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Mutable vs Immutable Objects

A mutable object can be changed after it's created, and an immutable object can't.

In Javascript, everything (except for strings) is mutable by default:

const array = [4, 9]; array[0] = 1; // array is now [1, 9]

Freezing an object makes it immutable, though:

const array = [4, 9]; // Make it immutable Object.freeze(array); array[0] = 1; // array is still [4, 9]

Strings can be mutable or immutable depending on the language.

Strings are immutable in Javascript:

const testString = 'mutable?'; testString[7] = '!'; // String is still 'mutable?' // (but no error is raised!)

But in some other languages, like Swift, strings can be mutable:

var testString = "mutable?" if let range = testString.range(of: "?") { testString.replaceSubrange(range, with: "!") // testString is now "mutable!" }

Mutable objects are nice because you can make changes in-place, without allocating a new object. But be careful—whenever you make an in-place change to an object, all references to that object will now reflect the change.

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