Interview Cake

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.

What's next?

If you're ready to start applying these concepts to some problems, check out our mock coding interview questions.

They mimic a real interview by offering hints when you're stuck or you're missing an optimization.

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