JavaScript variable assignment

Question

Maybe the question already exists, but I don't find anything because I don't know how to call this problem.

I have an object function and want to assign the object variable into a variable:

this.myFunction = function() {
    var selection;                // internal variable
    selection = this.selection;   // both are [0,0]

    console.log(this.selection);  // result: [0,0]

    selection[0] = 500;           // set internal variable[0] 500
    console.log(selection);       // result: [500,0]
    console.log(this.selection);  // result: [500,0] <-- expected: [0,0]
}

How I can change the local variable without changing the other?

javascript assignment is always by reference for objects, if you want also the reason why — Kaddath, Feb 13 '17 at 14:42
`Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations.` This means whenever you assign an array to another variable, it's just a mutator. It's not a deep clone. — Sterling Archer, Feb 13 '17 at 14:43
@Kaddath: that's a common misunderstanding and not what *by reference* means. JavaScript is *pass by value*, but the value is *a* reference (to an object). — Felix Kling, Feb 13 '17 at 14:44
In JS, Objects (and arrays are objects) are passed by _reference_ -- as such, selection and this.selection _refer to the same array_. If you want them to be different you'll have to do as other commenters have suggested and make a copy of the array ([array.slice](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice) should be a good option to do that). — Alexander Nied, Feb 13 '17 at 14:44
Not directly related to your question/problem, but what do you think `this` in `this.myFunction` means? What do you think `this` in `this.selection` means? — , Feb 13 '17 at 14:45
@FelixKling - sure, fair point, this is discussed [here](http://stackoverflow.com/questions/13104494/does-javascript-pass-by-reference#13104500). I was simply using a common shorthand to help illustrate the point. — Alexander Nied, Feb 13 '17 at 14:47
@torazaburo it means that anything done to the array reflects on all references -- aka mutative. All native array methods mutate the array, they do not create clones. — Sterling Archer, Feb 13 '17 at 15:20

score 2 · Accepted Answer · edited May 23 '17 at 12:01

2

When you call selection = this.selection, you are copying the reference of this.selection value. So when you change the local variable selection, you are changing this.selection too.

You have to use the method slice() to create a new array from the value. Like this:

selection = this.selection.slice();

edited May 23 '17 at 12:01

Community

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answered Feb 13 '17 at 14:42

Diego Faria

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`selection = this.selection.slice()` works thanks – Hendel Jo Feb 13 '17 at 14:47

JavaScript variable assignment

1 Answers1