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89

Understanding Javascript immutable variable

rated 0 times [ 96] [ 7] / answers: 1 / hits: 32459 / 11 Years ago, sat, april 20, 2013, 12:00:00

I am trying to understand what a Javascript immutable variable means. If I can do:

var x = astring;

x = str;

console.log(x); //logs str` , then why it is immutable?

The only answer I can think (from the little bit of C I know) is that var x is a pointer to a memory block with the value astring, and after the 2nd statement it points to another block with the value str. Is that the case?

And a bonus question: I was confused by the value types of Javascript. Are all variables objects under the hood? Even number and strings?

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answered 11 Years ago abagail · Accepted Answer

Values are immutable; variables are not; they hold a reference to their (primitive) values.

The three primitive types string, number and boolean have corresponding types whose instances are objects: String, Number, Boolean.

They are sometimes called wrapper types.

The following values are primitive:

Strings: hello

Numbers: 6, 3.14 (all numbers in JavaScript are floating point)

Booleans: true, false

null: usually explicitly assigned

undefined: usually the default (automatically assigned) value

All other values are objects, including wrappers for primitives.

So:

Objects are mutable by default

Objects have unique identities and are compared by reference

Variables hold references to objects

Primitives are immutable

Primitives are compared by value, they don’t have individual identities

You might find The Secret Life of JavaScript Primitives a good explanation.

Also, in ES6 there is a new const keyword, that creates a read-only named constant that cannot change value through assignment or be re-declared while the script is running.

Hope this helps!