Which equals operator (== vs ===) should be used in JavaScript comparisons?
I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace
== (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing
== with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over
==?Answer:
The identity (
===) operator behaves identically to the equality (==) operator except no type conversion is done, and the types must be the same to be considered equal.
Reference: Javascript Tutorial: Comparison Operators
The
== operator will compare for equality after doing any necessary type conversions. The ===operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators:===and!==, and their evil twins==and!=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then===producestrueand!==producesfalse. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:'' == '0' // false 0 == '' // true 0 == '0' // true false == 'false' // false false == '0' // true false == undefined // false false == null // false null == undefined // true ' \t\r\n ' == 0 // trueThe lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use===and!==. All of the comparisons just shown producefalsewith the===operator.
Update:
A good point was brought up by @Casebash in the comments and in @Phillipe Laybaert's answer concerning reference types. For reference types
== and === act consistently with one another (except in a special case).var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a literal with an object that evaluates to the same literal, due to its
toString or valueOf method. For example, consider the comparison of a string literal with a string object created by the String constructor."abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the
== operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects.
http://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons
