Comparison Operators & Equality

Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=. 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.

    // bad
if (isValid === true) {
  // ...
}

// good
if (isValid) {
  // ...
}

// bad
if (name) {
  // ...
}

// good
if (name !== '') {
  // ...
}

// bad
if (collection.length) {
  // ...
}

// good
if (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 that contain lexical declarations (e.g. let, const, function, and class).

    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.

    eslint rules: no-case-declarations.

    // bad
switch (foo) {
  case 1:
    let x = 1;
    break;
  case 2:
    const y = 2;
    break;
  case 3:
    function f() {
      // ...
    }
    break;
  default:
    class C {}
}

// good
switch (foo) {
  case 1: {
    let x = 1;
    break;
  }
  case 2: {
    const y = 2;
    break;
  }
  case 3: {
    function f() {
      // ...
    }
    break;
  }
  case 4:
    bar();
    break;
  default: {
    class C {}
  }
}

  • 15.6 Ternaries should not be nested and generally be single line expressions.

    eslint rules: no-nested-ternary.

    // bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// better
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull;

// best
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

  • 15.7 Avoid unneeded ternary statements.

    eslint rules: no-unneeded-ternary.

    // bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;

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