Front End Engineering, May 2015

Equality (==)

• If two values have the same type, just compare them
• If two values have different types, try to coerce them into the same type and then compare them.

Coersion

• When comparing a Number and String, coerce the String to a Number
• When comparing a Boolean and Anything, coerce the Boolean to a Number
• null == undefined // => true
• NaN == NaN // => false

• There can be multiple steps

  /* 1 */ 1 == "" // coerce String -> Number
  /* 2 */ 1 == 0 // => false
  
  /* 1 */ "1" == true // coerce Boolean -> Number
  /* 2 */ "1" == 1 // coerce String -> Number
  /* 3 */ 1 == 1 // => true
  

Strict equality

When using the strict equality operator (===), types are not coerced.

To put it another way, two expressions are strictly equal if they have the same type and the same value.

Object equality

Objects (including Arrays and Functions) are compared by reference. Two references are equal only if they reference the same object.

[1] === [1] // => false

var a = [1];
var b = a; // b is now a reference to the same object as a
a === b // => true

Other coersion

• If you try to add anything to a String, the other operand is coerced to a String and the two Strings are concatenated
• To coerce an Object (including Arrays and Functions) to String, the .toString method is called on the Object