Comparison Operators & Equality

15.1 Use === and !== over == and !=.
15.2 Conditional statements such as the if statement evaulate 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 is an object, objects evaluate to true
}

15.3 Use shortcuts.

// bad
if (name !== '') {
  // ...stuff...
}

// good
if (name) {
  // ...stuff...
}

// bad
if (collection.length > 0) {
  // ...stuff...
}

// good
if (collection.length) {
  // ...stuff...
}

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