Skip to content
This repository has been archived by the owner on Jul 4, 2024. It is now read-only.

Eslint rules

T. Baz edited this page Aug 23, 2018 · 3 revisions

Note: We used Airbnb JavaScript Style Guide for the most of rules, examples and explanations.

Eslint rules

Table of Contents

  1. References
  2. Objects
  3. Arrays
  4. Strings
  5. Functions
  6. Arrow Functions
  7. Classes & Constructors
  8. Modules
  9. Iterators and Generators
  10. Properties
  11. Variables
  12. Comparison Operators & Equality
  13. Blocks
  14. Comments
  15. Whitespace
  16. Commas
  17. Semicolons
  18. Naming Conventions

References

  • 1.1 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;

  • 1.2 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;
    }

⬆ back to top

Objects

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

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

⬆ back to top

Arrays

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

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

  • 3.2 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 7.2. 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;
    });

⬆ back to top

Strings

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

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

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

    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();
    }
    
    // good
    function sayHi(name) {
      return `How are you, ${ name }?`;
    }
    
    // good
    function sayHi(name) {
      return `How are you, ${name}?`;
    }

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

  • 4.4 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}'`;

⬆ back to top

Functions

  • 5.1 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)
    (function () {
      console.log('Welcome to the Internet. Please follow me.');
    }());

  • 5.2 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

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

    // bad
    if (currentUser) {
      function test() {
        console.log('Nope.');
      }
    }
    
    // good
    let test;
    if (currentUser) {
      test = () => {
        console.log('Yup.');
      };
    }

  • 5.4 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.

    // bad
    var add = new Function('a', 'b', 'return a + b');
    
    // still bad
    var subtract = Function('a', 'b', 'return a - b');

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

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

    // bad
    const f = function(){};
    const f = function (){};
    
    // good
    const f = function() {};
    const f = function () {};

  • 5.6 Never mutate parameters. eslint: no-param-reassign rule warn

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

    // bad
    function f1(obj) {
      obj.key = 1;
    }
    
    // good
    function f2(obj) {
      const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    }

  • 5.7 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.

    // bad
    function f1(a) {
      a = 1;
      // ...
    }
    
    function f2(a) {
      if (!a) { a = 1; }
      // ...
    }
    
    // good
    function f3(a) {
      const b = a || 1;
      // ...
    }
    
    function f4(a = 1) {
      // ...
    }

⬆ back to top

Arrow Functions

  • 6.1 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) {
      const y = x + 1;
      return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
      const y = x + 1;
      return x * y;
    });

  • 6.2 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 => {
      const nextNumber = number + 1;
      `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map(number => `A string containing the ${number}.`);
    
    // good
    [1, 2, 3].map((number) => {
      const nextNumber = number + 1;
      return `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number, index) => ({
      [index]: number,
    }));
    
    // No implicit return with side effects
    function foo(callback) {
      const val = callback();
      if (val === true) {
        // Do something if callback returns true
      }
    }
    
    let bool = false;
    
    // bad
    foo(() => bool = true);
    
    // good
    foo(() => {
      bool = true;
    });

  • 6.3 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 => {
      const y = x + 1;
      return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
      const y = x + 1;
      return x * y;
    });

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

    // bad
    const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;
    
    // bad
    const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;
    
    // good
    const itemHeight = item => (item.height > 256 ? item.largeSize : item.smallSize);
    
    // good
    const itemHeight = (item) => {
      const { height, largeSize, smallSize } = item;
      return height > 256 ? largeSize : smallSize;
    };

  • 6.5 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
    )

⬆ back to top

Classes & Constructors

  • 7.1 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.

    // bad
    class Foo {
      bar() { return 1; }
      bar() { return 2; }
    }
    
    // good
    class Foo {
      bar() { return 1; }
    }
    
    // good
    class Foo {
      bar() { return 2; }
    }

⬆ back to top

Modules

  • 8.1 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.

    // bad
    import foo from 'foo';
    // … some other imports … //
    import { named1, named2 } from 'foo';
    
    // good
    import foo, { named1, named2 } from 'foo';
    
    // good
    import foo, {
      named1,
      named2,
    } from 'foo';

⬆ back to top

Iterators and Generators

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

    Why? This enforces our immutable rule. 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.

    const numbers = [1, 2, 3, 4, 5];
    
    // bad
    let sum = 0;
    for (let num of numbers) {
      sum += num;
    }
    sum === 15;
    
    // good
    let sum = 0;
    numbers.forEach((num) => {
      sum += num;
    });
    sum === 15;
    
    // best (use the functional force)
    const sum = numbers.reduce((total, num) => total + num, 0);
    sum === 15;
    
    // bad
    const increasedByOne = [];
    for (let i = 0; i < numbers.length; i++) {
      increasedByOne.push(numbers[i] + 1);
    }
    
    // good
    const increasedByOne = [];
    numbers.forEach((num) => {
      increasedByOne.push(num + 1);
    });
    
    // best (keeping it functional)
    const increasedByOne = numbers.map(num => num + 1);

Properties

  • 10.1 Use dot notation when accessing properties. eslint: dot-notation rule warn

    const luke = {
      jedi: true,
      age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;

⬆ back to top

Variables

  • 11.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const

    // bad
    superPower = new SuperPower();
    
    // good
    const superPower = new SuperPower();

  • 11.2 Use one const or let declaration per variable. 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.

    // bad
    const items = getItems(),
        goSportsTeam = true,
        dragonball = 'z';
    
    // bad
    // (compare to above, and try to spot the mistake)
    const items = getItems(),
        goSportsTeam = true;
        dragonball = 'z';
    
    // good
    const items = getItems();
    const goSportsTeam = true;
    const dragonball = 'z';

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

    Why? Chaining variable assignments creates implicit global variables.

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

  • 11.4 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.

    // bad
    const foo =
      superLongLongLongLongLongLongLongLongFunctionName();
    
    // bad
    const foo
      = 'superLongLongLongLongLongLongLongLongString';
    
    // good
    const foo = (
      superLongLongLongLongLongLongLongLongFunctionName()
    );
    
    // good
    const foo = 'superLongLongLongLongLongLongLongLongString';

  • 11.5 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.

    // bad
    
    var some_unused_var = 42;
    
    // Write-only variables are not considered as used.
    var y = 10;
    y = 5;
    
    // A read for a modification of itself is not considered as used.
    var z = 0;
    z = z + 1;
    
    // Unused function arguments.
    function getX(x, y) {
        return x;
    }
    
    // good
    
    function getXPlusY(x, y) {
      return x + y;
    }
    
    var x = 1;
    var y = 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.
    var { type, ...coords } = data;
    // 'coords' is now the 'data' object without its 'type' property.

⬆ back to top

Comparison Operators & Equality

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

    // bad
    const foo = maybe1 > maybe2
      ? "bar"
      : value1 > value2 ? "baz" : null;
    
    // split into 2 separated ternary expressions
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    // better
    const foo = maybe1 > maybe2
      ? 'bar'
      : maybeNull;
    
    // best
    const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

  • 12.2 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.

    // bad
    const foo = a && b < 0 || c > 0 || d + 1 === 0;
    
    // bad
    const bar = a ** b - 5 % d;
    
    // bad
    // one may be confused into thinking (a || b) && c
    if (a || b && c) {
      return d;
    }
    
    // good
    const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
    
    // good
    const bar = (a ** b) - (5 % d);
    
    // good
    if (a || (b && c)) {
      return d;
    }
    
    // good
    const bar = a + b / c * d;

⬆ back to top

Blocks

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

    // bad
    if (test)
      return false;
    
    // good
    if (test) return false;
    
    // good
    if (test) {
      return false;
    }
    
    // bad
    function foo() { return false; }
    
    // good
    function bar() {
      return false;
    }

  • 13.2 If you're using multi-line blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

    // bad
    if (test) {
      thing1();
      thing2();
    }
    else {
      thing3();
    }
    
    // good
    if (test) {
      thing1();
      thing2();
    } else {
      thing3();
    }

  • 13.3 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 rule warn

    // bad
    function foo() {
      if (x) {
        return x;
      } else {
        return y;
      }
    }
    
    // bad
    function cats() {
      if (x) {
        return x;
      } else if (y) {
        return y;
      }
    }
    
    // bad
    function dogs() {
      if (x) {
        return x;
      } else {
        if (y) {
          return y;
        }
      }
    }
    
    // good
    function foo() {
      if (x) {
        return x;
      }
    
      return y;
    }
    
    // good
    function cats() {
      if (x) {
        return x;
      }
    
      if (y) {
        return y;
      }
    }
    
    // good
    function dogs(x) {
      if (x) {
        if (z) {
          return y;
        }
      } else {
        return z;
      }
    }

⬆ back to top

Comments

  • 14.1 Use /** ... */ for multi-line comments. eslint: multiline-comment-style

    // bad
    // make() returns a new element
    // based on the passed in tag name
    //
    // @param {String} tag
    // @return {Element} element
    function make(tag) {
    
      // ...
    
      return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }

  • 14.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block. eslint: multiline-comment-style

    // bad
    const active = true;  // is current tab
    
    // good
    // is current tab
    const active = true;
    
    // bad
    function getType() {
      console.log('fetching type...');
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }
    
    // good
    function getType() {
      console.log('fetching type...');
    
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }
    
    // also good
    function getType() {
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }

  • 14.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

    // bad
    //is current tab
    const active = true;
    
    // good
    // is current tab
    const active = true;
    
    // bad
    /**
     *make() returns a new element
     *based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }

⬆ back to top

Whitespace

  • 15.1 Use soft tabs (space character) set to 4 spaces. eslint: indent

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

  • 15.2 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',
    });

  • 15.3 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!');
    }

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

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

  • 15.5 Do not add spaces inside parentheses. eslint: space-in-parens

    // bad
    function bar( foo ) {
      return foo;
    }
    
    // good
    function bar(foo) {
      return foo;
    }
    
    // bad
    if ( foo ) {
      console.log(foo);
    }
    
    // good
    if (foo) {
      console.log(foo);
    }

  • 15.6 Do not add spaces inside brackets. eslint: array-bracket-spacing

    // bad
    const foo = [ 1, 2, 3 ];
    console.log(foo[ 0 ]);
    
    // good
    const foo = [1, 2, 3];
    console.log(foo[0]);

  • 15.7 Add spaces inside curly braces. eslint: object-curly-spacing

    // bad
    const foo = {clark: 'kent'};
    
    // good
    const foo = { clark: 'kent' };

  • 15.8 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

    Why? This ensures readability and maintainability.

    // bad
    const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
    
    // bad
    $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
    
    // good
    const foo = jsonData
      && jsonData.foo
      && jsonData.foo.bar
      && jsonData.foo.bar.baz
      && jsonData.foo.bar.baz.quux
      && jsonData.foo.bar.baz.quux.xyzzy;
    
    // good
    $.ajax({
      method: 'POST',
      url: 'https://airbnb.com/',
      data: { name: 'John' },
    })
      .done(() => console.log('Congratulations!'))
      .fail(() => console.log('You have failed this city.'));

  • 15.9 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing

    // bad
    function foo() {return true;}
    if (foo) { bar = 0;}
    
    // good
    function foo() { return true; }
    if (foo) { bar = 0; }

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

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

  • 15.11 Enforce spacing inside of computed properties. eslint: computed-property-spacing

    // bad
    obj[foo ]
    obj[ 'foo']
    var x = {[ b ]: a}
    obj[foo[ bar ]]
    
    // good
    obj[foo]
    obj['foo']
    var x = { [b]: a }
    obj[foo[bar]]

  • 15.12 Enforce spacing between functions and their invocations. eslint: func-call-spacing

    // bad
    func ();
    
    func
    ();
    
    // good
    func();

  • 15.13 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

    // bad
    var obj = { "foo" : 42 };
    var obj2 = { "foo":42 };
    
    // good
    var obj = { "foo": 42 };

  • 15.15 Avoid multiple empty lines and only allow one newline at the end of files. eslint: no-multiple-empty-lines

    // bad
    var x = 1;
    
    
    
    var y = 2;
    
    // good
    var x = 1;
    
    var y = 2;

⬆ back to top

Commas

  • 16.1 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'
    };

⬆ back to top

Semicolons

  • 17.1 Yup. eslint: semi

    Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

    // bad - raises exception
    const luke = {}
    const leia = {}
    [luke, leia].forEach(jedi => jedi.father = 'vader')
    
    // bad - raises exception
    const reaction = "No! That's impossible!"
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }())
    
    // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
    function foo() {
      return
        'search your feelings, you know it to be foo'
    }
    
    // good
    const luke = {};
    const leia = {};
    [luke, leia].forEach((jedi) => {
      jedi.father = 'vader';
    });
    
    // good
    const reaction = "No! That's impossible!";
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }());
    
    // good
    function foo() {
      return 'search your feelings, you know it to be foo';
    }

    Read more.

⬆ back to top

Naming Conventions

  • 18.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length rule warn

    // bad
    function q() {
      // ...
    }
    
    // good
    function query() {
      // ...
    }

  • 18.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

    // bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c() {}
    
    // good
    const thisIsMyObject = {};
    function thisIsMyFunction() {}

  • 18.3 Use PascalCase only when naming constructors or classes. eslint: new-cap rule warn

    // warn
    function user(options) {
      this.name = options.name;
    }
    
    const bad = new user({
      name: 'nope',
    });
    
    // good
    class User {
      constructor(options) {
        this.name = options.name;
      }
    }
    
    const good = new User({
      name: 'yup',
    });

  • 18.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle rule warn

    Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.

    // bad
    this.__firstName__ = 'Panda';
    this.firstName_ = 'Panda';
    this._firstName = 'Panda';
    
    // good
    this.firstName = 'Panda';
    
    // good, in environments where WeakMaps are available
    // see https://kangax.github.io/compat-table/es6/#test-WeakMap
    const firstNames = new WeakMap();
    firstNames.set(this, 'Panda');

React eslint rules

To see our react rules


⬆ back to top

Contributing

Open a Pull Request updating the .eslintrc.json. Also, update the README.md following:

Topic