✨ Now with the new ESLint Flat Config Format! ✨

A mostly reasonable approach to JavaScript

_{Or... Mainly NodeJS...}

• Make sure ESLint is installed. For VSCode, download the extension here
• Navigate inside your project
• Install Babel-ESLint*:
  $ npm i -D @babel/eslint-parser @babel/core eslint @types/node
  Or with Bun:
  $ bun i -D @babel/eslint-parser @babel/core eslint @types/bun
• Get the config:
  $ wget https://raw.githubusercontent.com/NullDev/JavaScript-Styleguide/master/eslint.config.js
• Done! :)

_{*) The rationale for using Babel-ESLint is that it supports the newest Stage-3 ECMAScript features.}

... with already preset config files and project structure

DOWNLOAD

$ npm i - and you're ready to go!

1. 📋 Types

   View contents

   • 1.1 Primitives
   • 1.2 Complex

2. 🔗 References

   View contents

   • 2.1 Prefer const
   • 2.2 Disallow var
   • 2.3 Block scope

3. 📦 Objects

   View contents

   • 3.1 Literals
   • 3.2 Computed properties
   • 3.3 Object shorthand
   • 3.4 Object concise
   • 3.5 Grouped shorthand
   • 3.6 Quoted properties
   • 3.7 Prototype Builtins
   • 3.8 Rest spread

4. 📑 Arrays

   View contents

   • 4.1 Literals
   • 4.2 Push
   • 4.3 Array spreads
   • 4.4 spread for iterable
   • 4.5 Array-like objects
   • 4.6 Mapping
   • 4.7 Callback return
   • 4.8 Linebreaks

5. 🔨 Destructuring

   View contents

   • 5.1 Object destructuring
   • 5.2 Array destructuring
   • 5.3 Object over array

6. 📄 Strings

   View contents

   • 6.1 Quotes
   • 6.2 Length
   • 6.3 Template literals
   • 6.4 Eval
   • 6.5 Escaping
   • 6.6 Regular Expressions (RegEx)

7. 📟 Functions

   View contents

   • 7.1 Strict mode
   • 7.2 Declarations
   • 7.3 IIFE's
   • 7.4 Non-function blocks
   • 7.5 Blocks (Info)
   • 7.6 Arguments shadow
   • 7.7 Rest
   • 7.8 Default parameters
   • 7.9 Side effects
   • 7.10 Defaults last
   • 7.11 Function constructor
   • 7.12 Signature spacing
   • 7.13 Mutating parameters
   • 7.14 Reassigning parameters
   • 7.15 Spread
   • 7.16 Signature invocation indentation

8. ➡️ Arrow Functions

   View contents

   • 8.1 Usage
   • 8.2 Implicit return
   • 8.3 Wrap
   • 8.4 Omitting parentheses
   • 8.5 Confusing functions
   • 8.6 Linebreaks

9. 📐 Classes & Constructors

   View contents

   • 9.1 Use class
   • 9.2 Inheritance
   • 9.3 Chaining
   • 9.4 toString methods
   • 9.5 No empty constructors
   • 9.6 No duplicate members
   • 9.7this in Class-Methods

10. 📮 Modules

    View contents

    • 10.1 CommonJS
    • 10.2 Duplicate imports
    • 10.3 Mutable exports
    • 10.4 Imports first
    • 10.5 Imports extensions
    • 10.6 Multiline imports

11. 🔃 Iterators and Generators

    View contents

    • 11.1 Higher-order functions
    • 11.2 Generators
    • 11.3 Spacing

12. 📑 Properties

    View contents

    • 12.1 Dot notation
    • 12.2 Bracket notation
    • 12.3 Exponentiation operator

13. 💾 Variables

    View contents

    • 13.1 Global namespace
    • 13.2 Declaration per variable
    • 13.3 Grouping
    • 13.4 Placing
    • 13.5 Chain assignment
    • 13.6 Increment & decrement
    • 13.7 Linebreaks
    • 13.8 Unused variables
    • 13.9 Bitwise floor

14. ⤴️Hoisting

    View contents

    • 14.1 About
    • 14.2 Anonymous function expressions
    • 14.3 Named function expressions
    • 14.4 Function declarations

15. ↔️Comparison Operators & Equality

    View contents

    • 15.1 Strict comparison
    • 15.2 Conditional statements
    • 15.3 Shortcuts
    • 15.4 Truth Equality
    • 15.5 Switch blocks
    • 15.6 Nested ternaries
    • 15.7 Unneeded-ternaries
    • 15.8 Mixed operators

16. 🔲 Blocks

    View contents

    • 16.1 Braces
    • 16.2 Cuddled elses
    • 16.3 Returns

17. 🔧 Control Statements

    View contents

    • 17.1 Length
    • 17.2 Selection operators

18. 📝 Comments

    View contents

    • 18.1 Language
    • 18.2 Multi-line
    • 18.3 Single-line
    • 18.4 Spaces
    • 18.5 Prefixing / Action-items
    • 18.6 FixMe
    • 18.7 ToDo

19. ⚪ Whitespace

    View contents

    • 19.1 Soft tabs
    • 19.2 Space Before blocks
    • 19.3 Around keywords
    • 19.4 Infix operators
    • 19.5 LF Line-breaks
    • 19.6 End of file
    • 19.7 Chains
    • 19.8 After blocks
    • 19.9 Padded blocks
    • 19.10 Inside parentheses
    • 19.11 Inside brackets
    • 19.12 Inside curly braces
    • 19.13 Line length
    • 19.14 Block spacing
    • 19.15 Comma spacing
    • 19.16 Computed properties
    • 16.17 Function call spacing
    • 19.18 Key spacing
    • 19.19 No Trailing spaces
    • 19.20 Multiple empty lines

20. 🔻 Commas

    View contents

    • 20.1 Leading commas
    • 20.2 Comma Dangling

21. ❗ Semicolons

    View contents

    • 21.1 Use semicolons

22. 🔀 Type Casting & Coercion

    View contents

    • 22.1 Explicit coercion
    • 22.2 Strings
    • 22.3 Numbers
    • 22.4 Deviations
    • 22.5 Bitwise
    • 22.6 Booleans
    • 22.7 Valid Typeof

23. 📜 Naming Conventions

    View contents

    • 23.0 Language
    • 23.1 Descriptive
    • 23.2 camelCase
    • 23.3 PascalCase
    • 23.4 Underscores
    • 23.5this
    • 23.6 Filename
    • 23.7 Default export
    • 23.8 Singleton
    • 23.9 Acronyms and initialisms
    • 23.10 Uppercase
    • 23.11 State-Booleans

24. ⬇️ Accessors

    View contents

    • 24.1 Not required accessors
    • 24.2 Getters & Setters
    • 24.3 Boolean prefix
    • 24.4 Consistency

25. 🔆 Events

    View contents

    • 25.1 Hash

26. 📖 Standard Library

    View contents

    • 26.1 isNaN
    • 26.2 isFinite

27. 🔩 jQuery

    View contents

    • 27.1 Prefix
    • 27.2 Cache
    • 27.3 DOM queries
    • 27.4 Find
    • 27.5 Shorthands
    • 27.6 Don't bloat .ready()

28. 🔄 ECMAScript 5 Compatibility

    View contents

    • 28.1 Compatibility table

29. 6️⃣ ECMAScript 6+ (ES 2015+) Styles

    View contents

    • 29.1 ES6 Styles
    • 29.2 Proposals

30. 🔌 Testing

    View contents

    • 30.1 Tests

31. 📈 Performance

    View contents

    • 31.1 Performance

32. 📚 Resources

    View contents

    • 32.1 Learning ES6+
    • 32.2 Read This
    • 32.3 Tools
    • 32.4 Further Reading
    • 32.5 Books
    • 32.6 Blogs
    • 32.7 Podcasts

33. ©️ Copyright

    View contents

    • 33.1 Base
    • 33.2 License

34. ♻️ Amendments

    View contents

    • 34.1 Forking

• 1.1Primitives: When you access a primitive type you work directly on its value.

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol
  • bigint

  constfoo=1;letbar=foo;bar=9;console.log(foo,bar);// => 1, 9

  • Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don't support them natively.

• 1.2Complex: When you access a complex type you work on a reference to its value.

  • object
  • array
  • function

  constfoo=[1,2];constbar=foo;bar[0]=9;console.log(foo[0],bar[0]);// => 9, 9

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• 2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

  Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.

  // badvara=1;varb=2;// goodconsta=1;constb=2;

• 2.2 If you must reassign references, use let instead of var. eslint: no-var

  Why? let is block-scoped rather than function-scoped like var.

  // badvarcount=1;if(true){count+=1;}// goodletcount=1;if(true){count+=1;}

• 2.3 Note that both let and const are block-scoped.

  // const and let only exist in the blocks they are defined in.{leta=1;constb=1;varc=1;}console.log(a);// ReferenceError: a is not definedconsole.log(b);// ReferenceError: b is not definedconsole.log(c);// 1

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• 3.1 Use the literal syntax for object creation. eslint: no-new-object

  // badconstitem=newObject();// goodconstitem={};

• 3.2 Use computed property names when creating objects with dynamic property names.

  Why? They allow you to define all the properties of an object in one place.

  functiongetKey(k){return`a key named ${k}`;}// badconstobj={id: 5,name: "foo",};obj[getKey("enabled")]=true;// goodconstobj={id: 5,name: "foo",[getKey("enabled")]: true,};

• 3.3 Use object method shorthand. eslint: object-shorthand

  // badconstfoo={value: 1,addValue: function(value){returnfoo.value+value;}};// goodconstfoo={value: 1,addValue(value){returnfoo.value+value;}};

• 3.4 Use property value shorthand. eslint: object-shorthand

  Why? It is shorter and descriptive.

  constfoo="bar";// badconstobj={foo: foo,};// goodconstobj={ foo,};

• 3.5 Group your shorthand properties at the beginning of your object declaration.

  Why? It’s easier to tell which properties are using the shorthand.

  constfoo="bar";constbar="foo";// badconstobj={test: 1,key: 2, foo,abc: 3,xyz: 4, bar,};// goodconstobj={ foo, bar,test: 1,key: 2,abc: 3,xyz: 4,};

• 3.6 Only quote properties that are invalid identifiers. eslint: quote-props

  Why? In general it is subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

  // badconstbad={"foo": 3,"bar": 4,"data-foo": 5,};// goodconstgood={foo: 3,bar: 4,"data-foo": 5,};

• 3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

  Why? These methods may be shadowed by properties on the object in question - consider {hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

  // badobject.hasOwnProperty(key);// goodObject.prototype.hasOwnProperty.call(object,key);// best - Stage 3 Proposal:// https://github.com/tc39/proposal-accessible-object-hasownpropertyObject.hasOwn(object,key);

• 3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

  // very badconstoriginal={a: 1,b: 2};constcopy=Object.assign(original,{c: 3});// this mutates `original`deletecopy.a;// so does this// badconstoriginal={a: 1,b: 2};constcopy=Object.assign({},original,{c: 3});// copy =>{a: 1, b: 2, c: 3 }// goodconstoriginal={a: 1,b: 2};constcopy={ ...original,c: 3};// copy =>{a: 1, b: 2, c: 3 }const{ a, ...noA}=copy;// noA =>{b: 2, c: 3 }

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• 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

  // badconstitems=newArray();// goodconstitems=[];

• 4.2 Use Array#push instead of direct assignment to add items to an array.

  constsomeStack=[];// badsomeStack[someStack.length]="foobar";// goodsomeStack.push("foobar");

• 4.3 Use array spreads ... to copy arrays. (Read more...)

  // badconstlen=items.length;constitemsCopy=[];leti;for(i=0;i<len;i+=1){itemsCopy[i]=items[i];}// goodconstitemsCopy=[...items];

• 4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

  constfoo=document.querySelectorAll(".foo");// goodconstnodes=Array.from(foo);// bestconstnodes=[...foo];

• 4.5 Use Array.from for converting an array-like object to an array.

  constarrLike={0: "foo",1: "bar",2: "baz",length: 3};// badconstarr=Array.prototype.slice.call(arrLike);// goodconstarr=Array.from(arrLike);

• 4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

  // badconstbaz=[...foo].map(bar);// goodconstbaz=Array.from(foo,bar);

• 4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

  // good[1,2,3].map((x)=>{consty=x+1;returnx*y;});// good[1,2,3].map(x=>x+1);// bad - no returned value means `acc` becomes undefined after the first iteration[[0,1],[2,3],[4,5]].reduce((acc,item,index)=>{constflatten=acc.concat(item);});// good[[0,1],[2,3],[4,5]].reduce((acc,item,index)=>{constflatten=acc.concat(item);returnflatten;});// badinbox.filter((msg)=>{const{ subject, response }=msg;if(subject==="foo")returnresponse==="bar";elsereturnfalse;});// goodinbox.filter((msg)=>{const{ subject, response }=msg;if(subject==="foo")returnresponse==="bar";returnfalse;});

• 4.8 Use line breaks after open and before close array brackets if an array has multiple lines

  // badconstarr=[[0,1],[2,3],[4,5]];constobjectInArray=[{id: 1,},{id: 2,}];constnumberInArray=[1,2, ... ];// goodconstarr=[[0,1],[2,3],[4,5], ... ];constobjectInArray=[{id: 1,},{id: 2,},];constnumberInArray=[1,2, ... ];

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• 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

  Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

  // badfunctiongetFullName(user){constfirstName=user.firstName;constlastName=user.lastName;return`${firstName}${lastName}`;}// goodfunctiongetFullName(user){const{ firstName, lastName }=user;return`${firstName}${lastName}`;}// bestfunctiongetFullName({ firstName, lastName }){return`${firstName}${lastName}`;}

• 5.2 Use array destructuring. eslint: prefer-destructuring

  constarr=[1,2,3,4];// badconstfirst=arr[0];constsecond=arr[1];// goodconst[first,second]=arr;

• 5.3 Use object destructuring for multiple return values, not array destructuring.

  Why? You can add new properties over time or change the order of things without breaking call sites.

  // badfunctionprocessInput(input){// then a miracle occursreturn[left,right,top,bottom];}// the caller needs to think about the order of return dataconst[left,__,top]=processInput(input);// goodfunctionprocessInput(input){// then a miracle occursreturn{ left, right, top, bottom };}// the caller selects only the data they needconst{ left, top }=processInput(input);

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• 6.1 Use double quotes "" for strings instead of single quotes ''. eslint: quotes

  Why? While other Styleguides may enforce single quotes, they mostly do it because of consistency (in favor of older projects). Here are some reasons for using double quotes:
  

  • Double quotes eliminate the need to escape apostrophes: "I'm" vs 'I\'m'.
  • From a linguistic point of view, double quotes identify a passage of quoted text while single quotes are commonly used as a contraction.
  • Double quotes are used to define strings in many other languages. Single quotes are used to define char's in some.
  • JSON Strings are only valid with double quotes.

  // badconstname='foo bar';// bad - template literals should contain interpolation or newlinesconstname=`foo bar`;// goodconstname="foo bar";

• 6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

  Why? Broken strings are painful to work with and make code less searchable.

  // badconsterrorMessage="This is a super long error - lorem ipsum dolor \sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget \dolor. Aenean massa. Cum sociis natoque \penatibus et.";// badconsterrorMessage="This is a super long error - lorem ipsum dolor "+"sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget "+"dolor. Aenean massa. Cum sociis natoque "+"penatibus et.";// goodconsterrorMessage="This is a super long error - lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et.";

• 6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-templatetemplate-curly-spacing

  Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

  // badfunctionsayHi(name){return"How are you, "+name+"?";}// very badfunctionsayHi(name){return["How are you, ",name,"?"].join();}// bad as wellfunctionsayHi(name){return`How are you, ${name}?`;}// goodfunctionsayHi(name){return`How are you, ${name}?`;}

• 6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

• 6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

  Why? Backslashes harm readability, thus they should only be present when necessary.

  // badconstfoo='\'this\' \i\s \"quoted\"';// goodconstfoo="\"this\" is 'quoted'";constfoo=`my name is "${name}"`;

• 6.6 Do not split regular expressions, even if some parts are used multiple times. The only exception are computed RegEx'es.

  Why? It has a great impact on readability and can lead to extremely confusing code

  // badconstbaseSite="http(?:s?):\/\/website\.com\/";consttopic="(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)";constcomment="(?:(?::)comment(\d+))";constuploadsRegex=newRegExp(baseSite+topic,"gi");constcommentRegex=newRegExp(baseSite+topic+comment,"gi");constprofileRegex=newRegExp(baseSite+"user\/(\w+)","gi");// goodconstuploadsRegex=/http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)/gi;constcommentRegex=/http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)(?:(?::)comment(\d+))/gi;constprofileRegex=/http(?:s?):\/\/website\.com\/user\/(\w+)/gi;

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• 7.1 Write the "use strict"; directive in each and every script you code and put it on the first line to scope it globally. Also, make an empty line below it. eslint: strict

  Why? It is a good way to make your code safer. This is because strict mode doesn't allow the usage of dangerous features which could work in a way you wouldn't expect. Some things it does:
  

  • It disallows non-explicit global variables.
  • Silent failing assignments will throw errors instead.
  • It requires all property names in an object literal to be unique.
  • Function parameter names must be unique as well.
    
    

  Also you do not need to worry about browser compartibility. It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. Read more about strict mode and why you should use it.
  
  Note: Modules are exempt from this rule because they always run in strict-mode.

  0// bad1console.log("Foo");2letx=function(){3console.log("Bar");4};5---0// bad1console.log("Foo");2letx=function(){3"use strict";4// strict mode is enabled in the scope of this function only!5console.log("Bar");6};7---0"use strict";12// ^ Good3console.log("Foo");4letx=function(){5console.log("Bar");6};7

• 7.2 Use named function expressions instead of function declarations. eslint: func-style

  Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error's call stack. (Discussion)

  // badfunctionfoo(){// ...}// okayletfoo=function(){// ...};// good// lexical name distinguished from the variable-referenced invocation(s)letshort=functionlongUniqueMoreDescriptiveLexicalFoo(){// ...};

• 7.3 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

  Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

  // immediately-invoked function expression (IIFE)// bad!function(){/* ... */}();~function(){/* ... */}();-function(){/* ... */}();+function(){/* ... */}();voidfunction(){/* ... */}();// good(function(){console.log("I'm an IIFE");}());

• 7.4 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

• 7.5Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

  // badif(currentUser){functiontest(){console.log("Foo");}}// goodlettest;if(currentUser){test=()=>{console.log("Foo");};}

• 7.6 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

  // badfunctionfoo(name,options,arguments){// ...}// goodfunctionfoo(name,options,args){// ...}

• 7.7 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

  Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

  // badfunctionconcatenateAll(){constargs=Array.prototype.slice.call(arguments);returnargs.join("");}// goodfunctionconcatenateAll(...args){returnargs.join("");}

• 7.8 Use default parameter syntax rather than mutating function arguments.

  // really badfunctionhandleThings(opts){// No! We shouldn't mutate function arguments.// Double bad: if opts is falsy it'll be set to an object which may// be what you want but it can introduce subtle bugs.opts=opts||{};// ...}// still badfunctionhandleThings(opts){if(opts===void0){opts={};}// ...}// goodfunctionhandleThings(opts={}){// ...}

• 7.9 Avoid side effects with default parameters.

  Why? They are confusing to reason about.

  letb=1;// badfunctioncount(a=b++){console.log(a);}count();// 1count();// 2count(3);// 3count();// 3

• 7.10 Always put default parameters last.

  // badfunctionhandleThings(opts={},name){// ...}// goodfunctionhandleThings(name,opts={}){// ...}

• 7.11 Never use the Function constructor to create a new function. eslint: no-new-func

  Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

  // badletadd=newFunction("a","b","return a + b");// still badletsubtract=Function("a","b","return a - b");

• 7.12 Spacing in a function signature. eslint: space-before-function-parenspace-before-blocks

  Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

  // badconstf=function(){};constg=function(){};consth=function(){};// goodconstx=function(){};consty=functiona(){};

• 7.13 Never mutate parameters. eslint: no-param-reassign

  Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

  // badfunctionf1(obj){obj.key=1;}// goodfunctionf2(obj){constkey=Object.prototype.hasOwnProperty.call(obj,"key") ? obj.key : 1;}

• 7.14 Never reassign parameters. eslint: no-param-reassign

  Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8 (and therefore NodeJS as well).

  // badfunctionf1(a){a=1;// ...}functionf2(a){if(!a)a=1;// ...}// goodfunctionf3(a){constb=a||1;// ...}functionf4(a=1){// ...}

• 7.15 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

  Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

  // badconstx=[1,2,3,4,5];console.log.apply(console,x);// goodconstx=[1,2,3,4,5];console.log(...x);// badnew(Function.prototype.bind.apply(Date,[null,2016,8,5]));// goodnewDate(...[2016,8,5]);

• 7.16 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself and with a trailing comma on the last item. eslint: function-paren-newline

  // badfunctionfoo(bar,baz,quux){// ...}// goodfunctionfoo(bar,baz,quux, ... ){// ...}// badconsole.log(foo,bar,baz);// goodconsole.log(foo,bar,baz, ... );

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• 8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

  Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

  Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.

  // bad[1,2,3].map(function(x){consty=x+1;returnx*y;});// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});

• 8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

  Why? Syntactic sugar. It reads well when multiple functions are chained together.

  // bad[1,2,3].map(number=>{constnextNumber=number+1;`A string containing the ${nextNumber}.`;});// good[1,2,3].map(number=>`A string containing the ${number+1}.`);// good[1,2,3].map((number)=>{constnextNumber=number+1;return`A string containing the ${nextNumber}.`;});// good[1,2,3].map((number,index)=>({[index]: number,}));// No implicit return with side effectsfunctionfoo(callback){constval=callback();if(val===true){// Do something if callback returns true}}letbool=false;// badfoo(()=>bool=true);// goodfoo(()=>{bool=true;});

• 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

  Why? It shows clearly where the function starts and ends.

  // bad["get","post","put"].map(httpMethod=>Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName,httpMethod,));// good["get","post","put"].map(httpMethod=>(Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName,httpMethod,)));

• 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint. eslint: arrow-parens

  Why? Less visual clutter.

  // bad[1,2,3].map((x)=>x*x);// good[1,2,3].map(x=>x*x);// good[1,2,3].map(number=>(`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`));// bad[1,2,3].map(x=>{consty=x+1;returnx*y;});// good[1,2,3].map((x)=>{consty=x+1;returnx*y;});

• 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

  // badconstitemHeight=item=>item.height>256 ? item.largeSize : item.smallSize;// badconstitemHeight=(item)=>item.height>256 ? item.largeSize : item.smallSize;// goodconstitemHeight=item=>(item.height>256 ? item.largeSize : item.smallSize);// goodconstitemHeight=(item)=>{const{ height, largeSize, smallSize }=item;returnheight>256 ? largeSize : smallSize;};

• 8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

  // bad(foo)=>bar;(foo)=>(bar);// good(foo)=>bar;(foo)=>(bar);(foo)=>(bar)

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• 9.1 Always use class. Avoid manipulating prototype directly.

  Why? class syntax is more concise and easier to reason about.

  // badfunctionQueue(contents=[]){this.queue=[...contents];}Queue.prototype.pop=function(){constvalue=this.queue[0];this.queue.splice(0,1);returnvalue;};// goodclassQueue{constructor(contents=[]){this.queue=[...contents];}pop(){constvalue=this.queue[0];this.queue.splice(0,1);returnvalue;}}

• 9.2 Use extends for inheritance.

  Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

  // badconstinherits=require("inherits");functionPeekableQueue(contents){Queue.apply(this,contents);}inherits(PeekableQueue,Queue);PeekableQueue.prototype.peek=function(){returnthis.queue[0];};// goodclassPeekableQueueextendsQueue{peek(){returnthis.queue[0];}}

• 9.3 Methods can return this to help with method chaining.

  // badPlayer.prototype.jump=function(){this.jumping=true;returntrue;};Player.prototype.setHeight=function(height){this.height=height;};constfoo=newPlayer();foo.jump();// => truefoo.setHeight(20);// => undefined// goodclassPlayer{jump(){this.jumping=true;returnthis;}setHeight(height){this.height=height;returnthis;}}constfoo=newPlayer();foo.jump().setHeight(20);

• 9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

  classPlayer{constructor(options={}){this.name=options.name||"no name";}getName(){returnthis.name;}toString(){return`Player - ${this.getName()}`;}}

• 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

  // badclassPlayer{constructor(){}getName(){returnthis.name;}}// badclassFooextendsPlayer{constructor(...args){super(...args);}}// goodclassFooextendsPlayer{constructor(...args){super(...args);this.name="Foo";}}

• 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

  Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

  // badclassFoo{bar(){return1;}bar(){return2;}}// goodclassFoo{bar(){return1;}}// goodclassFoo{bar(){return2;}}

• 9.7 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

  // badclassFoo{bar(){console.log("bar");}}// good - this is usedclassFoo{bar(){console.log(this.bar);}}// good - constructor is exemptclassFoo{constructor(){// ...}}// good - static methods aren't expected to use thisclassFoo{staticbar(){console.log("bar");}}

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• 10.1 Stick to CommonJS Imports (RequireJS / module.exports).

  Why? Not many of the current browser engines implements import/export from the ES6 standard. In NodeJS, require() is still the standard way of importing modules. Plus, in NodeJS you can make use of dynamic module loading and Require's API's in general, where you can control caching as well.

  // badimportfoofrom"bar";exportdefaultbar;// badimport{foo}from"bar";exportdefaultbar;// goodletfoo=require("bar");module.exports=bar;

• 10.2 Only import from a path in one place. eslint: no-duplicate-imports

  Why? Having multiple lines that import from the same path can make code harder to maintain.

  // badletfoo=require("bar");// … some other imports … let{ foo2, foo3 }=require("bar");// goodlet{ foo, foo1, foo2 }=require("bar");// goodlet{ foo, foo1, foo2,}=require("bar");

• 10.3 Do not export mutable bindings. eslint: import/no-mutable-exports

  Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

  // badletfoo=3;module.exports=foo;// goodconstfoo=3;module.exports=foo;

• 10.4 Put all imports above non-import statements. eslint: import/first

  Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

  // badletfoo=require("foo");foo.init();letbar=require("bar");// goodletfoo=require("foo");letbar=require("bar");foo.init();

• 10.5 Do not include JavaScript filename extensions eslint: import/extensions

  Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

  // badimportfoofrom"./foo.js";importbarfrom"./bar.jsx";importbazfrom"./baz/index.jsx";// goodimportfoofrom"./foo";importbarfrom"./bar";importbazfrom"./baz";

• 10.6 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

  Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide.

  // badlet{ longNameA, longNameB, longNameC, longNameD, longNameE }=require("path");// goodlet{ longNameA, longNameB, longNameC, longNameD, longNameE,}=require("path");

• 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iteratorno-restricted-syntax

  Why? Dealing with pure functions that return values is easier to reason about than side effects.

  Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

  Note:for-in and for-of are mostly OK as well. Especially for small operations. Higher-order functions however are best practice.

  constnumbers=[1,2,3,4,5];// ok...letsum=0;for(letnumofnumbers){sum+=num;}sum===15;// goodletsum=0;numbers.forEach((num)=>{sum+=num;});sum===15;// best (use the functional force)constsum=numbers.reduce((total,num)=>total+num,0);sum===15;// badconstincreasedByOne=[];for(leti=0;i<numbers.length;i++){increasedByOne.push(numbers[i]+1);}// goodconstincreasedByOne=[];numbers.forEach((num)=>{increasedByOne.push(num+1);});// best (keeping it functional)constincreasedByOne=numbers.map(num=>num+1);

• 11.2 Use generators only in NodeJS for now.

  Why? They don’t transpile well to ES5.

• 11.3 If you must use generators, or if you disagree with 11.2, make sure their function signature is spaced properly. eslint: generator-star-spacing

  Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

  // badfunction*foo(){// ...}// badletbar=function*(){// ...};// badletbaz=function*(){// ...};// badletquux=function*(){// ...};// badfunction*foo(){// ...}// badfunction*foo(){// ...}// very badfunction*foo(){// ...}// very badletfoo=function*(){// ...};// goodfunction*foo(){// ...}// goodletfoo=function*(){// ...};

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• 12.1 Use dot notation when accessing properties. eslint: dot-notation

  constfoo={bar: true,baz: 5,};// badconstisTrue=foo["bar"];// goodconstisTrue=foo.bar;

• 12.2 Use bracket notation [] when accessing properties with a variable or if the key includes illegal characters.

  constfoo={bar: true,baz: 5,"test-1": "foo"};functiongetProp(prop){returnfoo[prop];}constisTrue=getProp("bar");constbar=foo["test-1"];

• 12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.

  // badconstbinary=Math.pow(2,10);// goodconstbinary=2**10;

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• 13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Variables declared without const, let or var are disallowed in strict mode as well. eslint: no-undefprefer-const

  // badfoo=newFoo();// goodconstfoo=newFoo();

• 13.2 Use one const or let declaration per variable or assignment. eslint: one-var

  Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

  // badconstitems=getItems(),foo=true,bar="baz";// bad// (compare to above, and try to spot the mistake)constitems=getItems(),foo=true;bar="baz";// goodconstitems=getItems();constfoo=true;constbar="baz";

• 13.3 Group all your consts and then group all your lets.

  Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

  // badleti,len,foo,items=getItems(),bar=true;// badleti;constitems=getItems();letfoo;constbar=true;letlen;// goodconstbar=true;constitems=getItems();letfoo;leti;letlength;

• 13.4 Assign variables where you need them, but place them in a reasonable place.

  Why? let and const are block scoped and not function scoped.

  // bad - unnecessary function callfunctioncheckName(hasName){constname=getName();if(hasName==="test")returnfalse;if(name==="test"){this.setName("");returnfalse;}returnname;}// goodfunctioncheckName(hasName){if(hasName==="test")returnfalse;constname=getName();if(name==="test"){this.setName("");returnfalse;}returnname;}

• 13.5 Don’t chain variable assignments. eslint: no-multi-assign

  Why? Chaining variable assignments creates implicit global variables.

  // bad(functionexample(){/** * JavaScript interprets this as * let a = ( b = ( c = 1 ) ); * The let keyword only applies to variable a; variables b and c become * global variables. */leta=b=c=1;}());console.log(a);// throws ReferenceErrorconsole.log(b);// 1console.log(c);// 1// good(functionexample(){leta=1;letb=a;letc=a;}());console.log(a);// throws ReferenceErrorconsole.log(b);// throws ReferenceErrorconsole.log(c);// throws ReferenceError// the same applies for `const`

• 13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

  Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

  // badconstarray=[1,2,3];letnum=1;num++;--num;letsum=0;lettruthyCount=0;for(leti=0;i<array.length;i++){letvalue=array[i];sum+=value;if(value)truthyCount++;}// goodconstarray=[1,2,3];letnum=1;num+=1;num-=1;constsum=array.reduce((a,b)=>a+b,0);consttruthyCount=array.filter(Boolean).length;

• 13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

  Why? Linebreaks surrounding = can obfuscate the value of an assignment.

  // badconstfoo=superLongLongLongLongLongLongLongLongFunctionName();// badconstfoo="superLongLongLongLongLongLongLongLongString";// goodconstfoo=(superLongLongLongLongLongLongLongLongFunctionName());// goodconstfoo="superLongLongLongLongLongLongLongLongString";

• 13.8 Disallow unused variables. eslint: no-unused-vars

  Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

  // badletsome_unused_var=42;// Write-only variables are not considered as used.lety=10;y=5;// A read for a modification of itself is not considered as used.letz=0;z=z+1;// Unused function arguments.functiongetX(x,y){returnx;}// goodfunctiongetXPlusY(x,y){returnx+y;}letx=1;lety=a+2;alert(getXPlusY(x,y));// 'type' is ignored even if unused because it has a rest property sibling.// This is a form of extracting an object that omits the specified keys.let{ type, ...coords}=data;// 'coords' is now the 'data' object without its 'type' property.

• 13.9 Do not floor variables with the bitwise or (x | 0). Use Math.floor() or if you must Math.trunc().

  Why? First off, it does notfloor the number. It truncates it (rounding towards 0). It causes odd Comparative behavior as well: Math.floor(NaN) === NaN, while (NaN | 0) === 0. Also, it works with 32-bit signed integers only. As mentioned above, use Math.trunc() if you have to. It is the ES5 equivalent of | 0 and it is able to work with numbers higher or equal to 2^31.

  letx=42.835// Badlety=x|0;// Okaylety=Math.trunc(x);// Good lety=Math.floor(x);

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• 14.1var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

  // this wouldn’t work (assuming there// is no notDefined global variable)functionexample(){console.log(notDefined);// => throws a ReferenceError}// creating a variable declaration after you// reference the variable will work due to// variable hoisting. Note: the assignment// value of `true` is not hoisted.functionexample(){console.log(declaredButNotAssigned);// => undefinedvardeclaredButNotAssigned=true;}// the interpreter is hoisting the variable// declaration to the top of the scope,// which means our example could be rewritten as:functionexample(){letdeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}// using const and letfunctionexample(){console.log(declaredButNotAssigned);// => throws a ReferenceErrorconsole.log(typeofdeclaredButNotAssigned);// => throws a ReferenceErrorconstdeclaredButNotAssigned=true;}

• 14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

  functionexample(){console.log(anonymous);// => undefinedanonymous();// => TypeError anonymous is not a functionvaranonymous=function(){console.log("anonymous function expression");};}

• 14.3 Named function expressions hoist the variable name, not the function name or the function body.

  functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionfoo();// => ReferenceError foo is not definedvarnamed=functionfoo(){console.log("bar");};}// the same is true when the function name// is the same as the variable name.functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionvarnamed=functionnamed(){console.log("named");};}

• 14.4 Function declarations hoist their name and the function body.

  functionexample(){foo();// => barfunctionfoo(){console.log("bar");}}

• For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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• 15.1 Use === and !== over == and != (Strict comparison). eslint: eqeqeq

• 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

  • Objects evaluate to true
  • Undefined evaluates to false
  • Null evaluates to false
  • Booleans evaluate to the value of the boolean
  • Numbers evaluate to false if +0, -0, or NaN, otherwise true
  • Strings evaluate to false if an empty string '', otherwise true

  if([0]&&[]){// true// an array (even an empty one) is an object, objects will evaluate to true}

• 15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

  // badif(isValid===true){// ...}// goodif(isValid){// ...}// badif(name){// ...}// goodif(name!==""){// ...}// badif(collection.length){// ...}// goodif(collection.length>0){// ...}

• 15.4 For more information see Truth Equality and JavaScript by Angus Croll.

• 15.5 Use braces to create blocks in case and default clauses. This is especially useful for those, which contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

  Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

  // badswitch(foo){case1: letx=1;break;case2: consty=2;break;case3: functionf(){// ...}break;default: classC{}}// goodswitch(foo){case1: {letx=1;break;}case2: {consty=2;break;}case3: {functionf(){// ...}break;}case4: {bar();break;}default: {classC{}}}

• 15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

  // badconstfoo=maybe1>maybe2 ? "bar" : value1>value2 ? "baz" : null;// split into 2 separated ternary expressionsconstmaybeNull=value1>value2 ? "baz" : null;// betterconstfoo=maybe1>maybe2 ? "bar" : maybeNull;// bestconstfoo=maybe1>maybe2 ? "bar" : maybeNull;

• 15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

  // badconstfoo=a ? a : b;constbar=c ? true : false;constbaz=c ? false : true;// goodconstfoo=a||b;constbar=!!c;constbaz=!c;

• 15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (+, -, *, & /) since their precedence is broadly understood. eslint: no-mixed-operators

  Why? This improves readability and clarifies the developer’s intention.

  // badconstfoo=a&&b<0||c>0||d+1===0;// badconstbar=a**b-5%d;// bad// one may be confused into thinking (a || b) && cif(a||b&&c){returnd;}// goodconstfoo=(a&&b<0)||c>0||(d+1===0);// goodconstbar=(a**b)-(5%d);// goodif(a||(b&&c)){returnd;}// goodconstbar=a+b/c*d;

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• 16.1 Use braces with all multi-line blocks. eslint: nonblock-statement-body-position

  Note: Use one-line statements only with short commands. Like if (x) y();
  They look nicer but can be hard to debug

  // badif(test)returnfalse;// goodif(test)returnfalse;// bad - it is too bulky and doesn't fit nicely in one rowif(test1)if(test2)stuff();// goodif(test1){if(test2)stuff();}// good (multiline)if(test){doStuff();// ...returnfalse;}// badfunctionfoo(){returnfalse;}// goodfunctionbar(){returnfalse;}

• 16.2 If you're using multi-line blocks with if and else, do not put else on the same line as your if block’s closing brace. eslint: brace-style

  // badif(test){thing1();thing2();}else{thing3();thing4();}// goodif(test){thing1();thing2();}else{thing3();thing4();}// bad if(test){thing1();thing2();}elsething3();// goodif(test){thing1();thing2();}elsething3();

• 16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

  // badfunctionfoo(){if(x)returnx;elsereturny;}// badfunctionbar(){if(x)returnx;elseif(y)returny;}// badfunctionbaz(){if(x)returnx;else{if(y)returny;}}// goodfunctionfoo(){if(x)returnx;returny;}// goodfunctionbar(){if(x)returnx;if(y)returny;}// goodfunctionbaz(x){if(x){if(z)returny;}elsereturnz;}

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• 17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

  Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

  // badif((foo===123||bar==="abc")&&doesItLookGoodWhenItBecomesThatLong()&&isThisReallyHappening()){thing1();}// badif(foo===123&&bar==="abc"){thing1();}// badif(foo===123&&bar==="abc"){thing1();}// badif(foo===123&&bar==="abc"){thing1();}// goodif(foo===123&&bar==="abc"){thing1();}// goodif((foo===123||bar==="abc")&&doesItLookGoodWhenItBecomesThatLong()&&isThisReallyHappening()){thing1();}// goodif(foo===123&&bar==="abc"){thing1();}