javascript operator “in”

Question

I\'m used to python, so

a = [1,2,3]
1 in a # -> True
b = [\"1\", \"2\", \"3\", \"x\"]
\"x\" in b # -> True

Why is it that in JavaScript

Answer 1

The thing you have to understand about JavaScript is almost everything is an "Object" that can have properties. Array's are just a special type of object whose properties are integer indexes and have push, pop, shift, unshift, etc. methods. Plus they can be defined with the square bracket shorthand you used:

a = [1,2,3];

This creates an Array object with the properties:

a[0] = 1;
a[1] = 2;
a[2] = 3;

Now as others have said, all the in operator does is check that an object has a property of that name and a[1] == 2 therefore 1 in a == true. On the other hand,

b = ["1", "2", "3", "x"];

created an Array object with the properties:

b[0] = "1";
b[1] = "2";
b[2] = "3";
b[3] = "x";

So b["x"] == undefined therefore "x" in b == false.

The other thing you have to understand is JavaScript uses "duck typing", meaning if it looks like a number, JavaScript treats it like a number. In this case, b["1"] == 2 therefore "1" in b == true. I'm not 100% certain whether this is duck typing at work or JavaScript just always treats property names as Strings.

If you wanted to declare a generic object that wouldn't have the Array methods but had the same properties you would write:

var b = {"0": "1", "1": "2", "2": "3", "3": "x"};

Which is shorthand for:

var b = {}; // This declares an Object
b[0] = "1";
b[1] = "2";
b[2] = "3";
b[3] = "x";