What's the reason to use === instead of == with typeof in Javascript?

Question

Throughout many third-party libraries and best practices blogs/recommendations, etc... it is common to see syntax like this:

typeof x === \'object\' (instead         


        

Answer 1

Triple equal operators are mostly used for variable type and value checking (all in 1 expression), also known as equality without type coercion.

Example:

var a = 1;
var b = 1;
var c = "1";
var d = "1";

alert (a === b); //True, same value and same type (numeric)
alert(c === d); //True, same value and same type (string)
alert(b === c); //False, different type but same value of 1

See Doug Crockford's YUI Theater on type coercion.

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If the typeof operator already returns a string, what's the need to type check the return value as well? If typeof(typeof(x)) is always string, no matter what x actually is, then == should be sufficient and === unnecessary.

The most efficient reason for not using typeof and rather the === operator would be for type coercion (interpretation) between browsers. Some browsers can pass 6=="6" as true and some wouldn't (depending on the strictness of the JS interpreter) so by introducing type coercion would clarify this. Also, it would bring the "Object-Orientativity" approach to it since JavasScript's variables are not type-based variables (i.e. variable types are not declared on compile time like in Java).

E.g. in Java, this would fail:

if ("6" instanceof Number) { // false

Hope I answered your question.