爱彼迎 js 规范

Comparison Operators & Equality

• 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.