Why 3 equal symbols in boolean comparisons? [duplicate]

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Javascr开发者_如何学编程ipt === vs == : Does it matter which “equal” operator I use?

Why do I see lots of javascript code lately with expressions that look like this:

if(val === "something")

Why "===" instead of just "=="? What's the difference? When should I use one or the other?

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The === does not allow type coercion, so something like this would return false:

if (2 === '2') // false

The "normal" javascript == operator does allow type coercion, and so this would return true:

if (2 == '2') // true

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var a = 3;
var b = "3";

if (a == b) {
  // this is true.
}

if (a === b) {
  // this is false.
}

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=== is typically referred to as the identity operator. Values being compared must be of the same type and value to be considered equal. == is typically referred to as the equality operator and performs type coercion to check equality.

An example

1 == '1' // returns true even though one is a number, the other a string

1 === '1' // returns false. different datatypes

Doug Crockford touches briefly on this in JavaScript the Good Parts google tech talk video. Worth spending an hour to watch.

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Checks that the type as well as the values match. This is important since (0 == false) is true but (0 === false) is not true.