Primitive values immutable or not?

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January 2nd, 2013, 09:04 AM

littlebigman

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Primitive values immutable or not?

Hello

I'm reading David Flanagan's "javascript: The Definitive Guide 6th ed", which says on page 44:

Quote:

There is a fundamental difference in JavaScript between primitive values (undefined, null, booleans, numbers, and strings) and objects (including arrays and functions). Primitives are immutable: there is no way to change (or “mutate”) a primitive value.

And still, the following code works:

Code:

"use strict";

var myvar = "hello, world!";
myvar = "changed!";

function myfunc(){
	console.log("Before");
	//alert(myvar);
	document.write(myvar);
	console.log("After");
}

I expected to get an error with "myvar="changed!";" but it works. Did I misunderstand what is meant with primitives being immutable?

Thank you.

January 2nd, 2013, 10:24 AM

AndrewSW

JavaScript is not spelt java

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Code:

var myvar = "hello, world!";
myvar = "changed!";

I know it seems that you are changing myvar but, technically, you are not. The 2nd line creates a new string that myvar now points to. The previous string "hello, world!" still exists (unaltered) in the computers' memory - but it's of no use as nothing points to it. (It will be removed from memory later by a process called garbage collection.)

Similarly,

Code:

var myvar = "Hello World!";
myvar = myvar + " Yo!";

does not modify myvar; the 2nd line (again) creates a new string. This is why string operations can be expensive.

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Kravvitz agrees!

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January 2nd, 2013, 10:35 AM

littlebigman

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Thanks for the info.

So primitive values cannot be changed in the sense that a new variable will be created on the fly, not in the sense that it would trigger an error as I expected.

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