Nisje JavaScript Style Guide

A mostly reasonable approach to JavaScript

Other Style Guides

• React

Table of Contents

1. Strings
2. References
3. Objects
4. Arrays
5. Commas
6. Whitespace

Strings

• 1.1 Use single quotes '' for strings. eslint: quotes

  // bad
  const name = "Capt. Kirk";
  
  // bad - template literals should contain interpolation or newlines
  const name = `Capt. Kirk`;
  
  // good
  const name = 'Capt. Kirk';

• 1.2 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.

  // bad
  const errorMessage = 'This is a super long error that was thrown because \
  of Batman. When you stop to think about how Batman had anything to do \
  with this, you would get nowhere \
  fast.';
  
  // bad
  const errorMessage = 'This is a super long error that was thrown because ' +
    'of Batman. When you stop to think about how Batman had anything to do ' +
    'with this, you would get nowhere fast.';
  
  // good
  const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

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

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

  // bad
  function sayHi( name ) {
    return 'How are you, ' + name + '?';
  }
  
  // bad
  function sayHi( name ) {
    return [ 'How are you, ', name, '?' ].join();
  }
  
  // bad
  function sayHi( name ) {
    return `How are you, ${name}?`;
  }
  
  // good
  function sayHi( name ) {
    return `How are you, ${ name }?`;
  }

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

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

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

  // bad
  const foo = '\'this\' \i\s \"quoted\"';
  
  // good
  const foo = '\'this\' is "quoted"';
  const foo = `my name is '${ name }'`;

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References

• 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.

  // bad
  var a = 1;
  var b = 2;
  
  // good
  const a = 1;
  const b = 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.

  // bad
  var count = 1;
  if ( true ) {
    count += 1;
  }
  
  // good, use the let.
  let count = 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.
  {
    let a = 1;
    const b = 1;
  }
  console.log( a ); // ReferenceError
  console.log( b ); // ReferenceError

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Objects

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

  // bad
  const item = new Object();
  
  // good
  const item = {};

• 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.

  function getKey( k ) {
    return `a key named ${ k }`;
  }
  
  // bad
  const obj = {
    id: 5,
    name: 'San Francisco',
  };
  obj[ getKey( 'enabled' ) ] = true;
  
  // good
  const obj = {
    id: 5,
    name: 'San Francisco',
    [ getKey( 'enabled' ) ]: true,
  };

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

  // bad
  const atom = {
    value: 1,
  
    addValue: function( value ) {
      return atom.value + value;
    },
  };
  
  // good
  const atom = {
    value: 1,
  
    addValue( value ) {
      return atom.value + value;
    },
  };

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

  Why? It is shorter and descriptive.

  const lukeSkywalker = 'Luke Skywalker';
  
  // bad
  const obj = {
    lukeSkywalker: lukeSkywalker,
  };
  
  // good
  const obj = {
    lukeSkywalker,
  };

• 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.

  const anakinSkywalker = 'Anakin Skywalker';
  const lukeSkywalker = 'Luke Skywalker';
  
  // bad
  const obj = {
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    lukeSkywalker,
    episodeThree: 3,
    mayTheFourth: 4,
    anakinSkywalker,
  };
  
  // good
  const obj = {
    lukeSkywalker,
    anakinSkywalker,
    episodeOne: 1,
    twoJediWalkIntoACantina: 2,
    episodeThree: 3,
    mayTheFourth: 4,
  };

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

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

  // bad
  const bad = {
    'foo': 3,
    'bar': 4,
    'data-blah': 5,
  };
  
  // good
  const good = {
    foo: 3,
    bar: 4,
    'data-blah': 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)).

  // bad
  console.log( object.hasOwnProperty( key ) );
  
  // good
  console.log( Object.prototype.hasOwnProperty.call( object, key ) );
  
  // best
  const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
  /* or */
  import has from 'has'; // https://www.npmjs.com/package/has
  // ...
  console.log( has.call( 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 bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign( original, { c: 3 } ); // this mutates `original` ಠ_ಠ
  delete copy.a; // so does this
  
  // bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign( {}, original, { c: 3 } ); // copy => { a: 1, b: 2, c: 3 }
  
  // good
  const original = { a: 1, b: 2 };
  const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
  
  const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

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Arrays

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

  // bad
  const items = new Array();
  
  // good
  const items = [];

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

  const someStack = [];
  
  // bad
  someStack[ someStack.length ] = 'abracadabra';
  
  // good
  someStack.push( 'abracadabra' );

• 4.3 Use array spreads ... to copy arrays.

  // bad
  const len = items.length;
  const itemsCopy = [];
  let i;
  
  for ( i = 0; i < len; i += 1 ) {
    itemsCopy[ i ] = items[ i ];
  }
  
  // good
  const itemsCopy = [ ...items ];

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

  const foo = document.querySelectorAll( '.foo' );
  
  // good
  const nodes = Array.from( foo );
  
  // best
  const nodes = [ ...foo ];

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

  const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };
  
  // bad
  const arr = Array.prototype.slice.call( arrLike );
  
  // good
  const arr = Array.from( arrLike );

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

  // bad
  const baz = [ ...foo ].map( bar );
  
  // good
  const baz = 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 ) => {
    const y = x + 1;
    return x * 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 ) => {
    const flatten = acc.concat( item );
    acc[ index ] = flatten;
  } );
  
  // good
  [ [ 0, 1 ], [ 2, 3 ], [ 4, 5 ] ].reduce( ( acc, item, index ) => {
    const flatten = acc.concat( item );
    acc[ index ] = flatten;
    return flatten;
  } );
  
  // bad
  inbox.filter( ( msg ) => {
    const { subject, author } = msg;
    if ( subject === 'Mockingbird' ) {
      return author === 'Harper Lee';
    } else {
      return false;
    }
  });
  
  // good
  inbox.filter( ( msg ) => {
    const { subject, author } = msg;
    if ( subject === 'Mockingbird' ) {
      return author === 'Harper Lee';
    }
  
    return false;
  } );

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

  // bad
  const arr = [
    [ 0, 1 ], [ 2, 3 ], [ 4, 5 ],
  ];
  
  const objectInArray = [ {
    id: 1,
  }, {
    id: 2,
  } ];
  
  const numberInArray = [
    1, 2,
  ];
  
  // good
  const arr = [ [ 0, 1 ], [ 2, 3 ], [ 4, 5 ] ];
  
  const objectInArray = [
    {
      id: 1,
    },
    {
      id: 2,
    },
  ];
  
  const numberInArray = [
    1,
    2,
  ];

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Commas

• 5.1 Leading commas: Nope. eslint: comma-style

  // bad
  const story = [
      once
    , upon
    , aTime
  ];
  
  // good
  const story = [
    once,
    upon,
    aTime,
  ];
  
  // bad
  const hero = {
      firstName: 'Ada'
    , lastName: 'Lovelace'
    , birthYear: 1815
    , superPower: 'computers'
  };
  
  // good
  const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace',
    birthYear: 1815,
    superPower: 'computers',
  };

• 5.2 Additional trailing comma: Yup. eslint: comma-dangle

  Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

  // bad - git diff without trailing comma
  const hero = {
       firstName: 'Florence',
  -    lastName: 'Nightingale'
  +    lastName: 'Nightingale',
  +    inventorOf: [ 'coxcomb chart', 'modern nursing' ]
  };
  
  // good - git diff with trailing comma
  const hero = {
       firstName: 'Florence',
       lastName: 'Nightingale',
  +    inventorOf: [ 'coxcomb chart', 'modern nursing' ],
  };

  // bad
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully'
  };
  
  const heroes = [
    'Batman',
    'Superman'
  ];
  
  // good
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully',
  };
  
  const heroes = [
    'Batman',
    'Superman',
  ];
  
  // bad
  function createHero(
    firstName,
    lastName,
    inventorOf
  ) {
    // does nothing
  }
  
  // good
  function createHero(
    firstName,
    lastName,
    inventorOf,
  ) {
    // does nothing
  }
  
  // good (note that a comma must not appear after a "rest" element)
  function createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  ) {
    // does nothing
  }
  
  // bad
  createHero(
    firstName,
    lastName,
    inventorOf
  );
  
  // good
  createHero(
    firstName,
    lastName,
    inventorOf,
  );
  
  // good (note that a comma must not appear after a "rest" element)
  createHero(
    firstName,
    lastName,
    inventorOf,
    ...heroArgs
  );

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Whitespace

• 6.1 Use tabs. eslint: indent

  // bad
  function foo() {
  ∙∙∙∙let name;
  }
  
  // good
  function foo() {
  »   let name;
  }

• 6.2 Place 1 space before the leading brace. eslint: space-before-blocks

  // bad
  function test(){
    console.log( 'test' );
  }
  
  // good
  function test() {
    console.log( 'test' );
  }
  
  // bad
  dog.set( 'attr',{
    age: '1 year',
    breed: 'Bernese Mountain Dog',
  } );
  
  // good
  dog.set( 'attr', {
    age: '1 year',
    breed: 'Bernese Mountain Dog',
  } );

• 6.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing

  // bad
  if( isJedi ) {
    fight ();
  }
  
  // good
  if ( isJedi ) {
    fight();
  }
  
  // bad
  function fight () {
    console.log ( 'Swooosh!' );
  }
  
  // good
  function fight() {
    console.log( 'Swooosh!' );
  }

• 6.4 Set off operators with spaces. eslint: space-infix-ops

  // bad
  const x=y+5;
  
  // good
  const x = y + 5;

• 6.5 End files with a single newline character. eslint: eol-last

  // bad
  import { es6 } from './styleguide';
    // ...
  export default es6;

  // bad
  import { es6 } from './styleguide';
    // ...
  export default es6;↵
  ↵

  // good
  import { es6 } from './styleguide';
    // ...
  export default es6;↵

• 6.6 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

  // bad
  const foo = 1,bar = 2;
  const arr = [ 1 , 2 ];
  
  // good
  const foo = 1, bar = 2;
  const arr = [ 1, 2 ];

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