The "in" operator in JavaScript

March 23, 2008

Javascript 1.5 contains the in operator that checks only property names. I can't figure out why Javascript returns the exact opposite of that other programming languages return on this operator. Here is an example:

> var hello = ["bonjour","hola","saluton","selam"];
> "bonjour" in hello
false
> 1 in hello
true

Actually, we don't need to the in operator to check values of Array, we can use indexOf property to this action easily;

> var hello = ["bonjour","hola","saluton","selam"];
> hello.indexOf("bonjour")>-1;
true
> hello.indexOf("Hallo")>-1;
false

This operator available to checking object properties too but there are many way to check object properties already;

> var hello = { "french":"bonjour", "esperanto":"saluton", "turkish":"selam" };
> "turkish" in hello
true
> "german" in hello
false
> Boolean(hello["turkish"]);
true
> Boolean(hello["german"]);
false