Javascript Object scope

Question

This is a simplification of something that I've come up against in a larger project so I hope it makes sense.

I have two Objects:

FirstObj = { data : [], init : function() { for(var a = 0; a < 20; a++) { this.data[a] = 1; } } }; SecondObj = { init : function() { var cell = FirstObj.data[0]; cell = 0; } };

I then call the two init methods and log the results:

(function(){ FirstObj.init(); SecondObj.init(); console.log(FirstObj.data); })()

Now, I was assuming - based on my basis in Actionscript - that the log would show an Array in which the first item is a 0 and the rest all 1 but the 0 does not seem to stick.

Why does the assignment of the 0 value not succeed here and yet works fine if, instead of cell = 0 I target directly at FirstObj.data[0] = 0.

I'm guessing this is a scope thing and I can work round it but I'm trying to get a proper understanding of how JS actually handles this stuff, especially when lumping code into Objects like this (as an aside, is this a good approach in peoples general opinion?).

Thank for any help.

Benjamin Gruenbaum · Accepted Answer · 2014-02-13 12:54:31Z

Numbers in JavaScript are something called primitive value types (alongside strings, booleans null and undefined).

This means, that when you do

var cell = FirstObj.data[0];

You're passing the value in FirstObj.data[0] and not a refernece to it.

What you're doing is like:

var x = 5; var y = x; // y is now 5 y = 4; // y is 4, x is still 5.

Of course, something like FirstObj.data[0] = 0 should work.

Frédéric Hamidi · Accepted Answer · 2014-02-13 12:54:24Z

Array indexing returns values in Javascript, not references. It means that once data[0] is assigned to cell, further modification of cell will not affect data[0].

Assigning the array itself would result in the behavior you're looking for:

SecondObj = { init : function() { var cells = FirstObj.data; cells[0] = 0; // Will also affect data[0]. } };

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