JavaScript Type System
JavaScript == vs ===
The double equals converts types before comparing; the triple equals does not.
This is one of the most asked JavaScript interview questions.
The short advice is simple: use === unless you have a reason not to.
Example
Example
javascript
console.log("5" == 5);
console.log("5" === 5);The output is true then false.
== converted the string; === compared the types too.
Loose and Strict Equality
== is loose equality: it converts types, then compares.
=== is strict equality: different types are never equal.
!= and !== are the matching opposites.
Syntax
Syntax
javascript
a == b // loose
a === b // strictPrefer strict equality by default.
Why Loose Equality Surprises People
The conversion rules produce some results nobody expects.
Example
Example
javascript
console.log(0 == "");
console.log(0 == "0");
console.log("" == "0");The first two are true and the last is false.
Equality here is not even consistent between the three.
Strict Equality Is Predictable
Different types are simply not equal, with no conversion at all.
Example
Example
javascript
console.log(0 === "");
console.log(0 === "0");
console.log(0 === 0);Only the last is true.
The One Useful Case for ==
null == undefined is true, and they are not equal to anything else.
So value == null is a neat check for either.
Example
Example
javascript
console.log(null == undefined);
console.log(null === undefined);
console.log(null == 0);The output is true, false, false.
Objects Compare by Identity
Two objects are equal only if they are the same object.
This is true for both operators.
Example
Example
javascript
const a = { id: 1 };
const b = { id: 1 };
console.log(a == b);
console.log(a === a);Contents are never compared, however alike they look.
Comparing Values You Do Not Control
Convert first, then compare strictly.
Example
Example
javascript
const entered = "42";
console.log(Number(entered) === 42);Your intention is obvious and the result is predictable.
Complete Example
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript == vs ===</title>
</head>
<body>
<h1>== vs ===</h1>
<p id="out"></p>
<script>
const lines = [
'"5" == 5 is ' + ("5" == 5),
'"5" === 5 is ' + ("5" === 5),
'0 == "" is ' + (0 == ""),
'0 === "" is ' + (0 === ""),
'null == undefined is ' + (null == undefined)
];
document.getElementById("out").innerHTML = lines.join("<br>");
</script>
</body>
</html>Try It Yourself
Run the above example in the Try It Editor.
Try more comparisons:
Add false == "0" and see what happens.
Important Points
==converts types before comparing.===compares the type as well as the value.- Loose equality gives inconsistent-looking results.
value == nullusefully catches null and undefined.- Objects are compared by identity, never by contents.
Conclusion
Use === by default and the surprises disappear.
The one exception, checking for null or undefined together, is worth knowing.
This is among the most reliably asked interview questions in JavaScript.