Stack vs heap | JavaScript Memory and Garbage Collection

Step 1 of 6

Two kinds of storage

JavaScript uses two areas of memory. The stack keeps track of function calls and their local variables. The heap stores objects. The garbage collector only cleans up the heap.

What to notice

The JavaScript spec never mentions a stack or a heap. They describe how engines like V8 work, and that's the model interviewers expect you to use.

The code

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// A user opens a question on devtools.tech.
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// number
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let questionId = 402;
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// boolean
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let isPremium = true;
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// string: its text lives on the heap
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let slug = "debounce";
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// stored on heap
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let question = {
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  slug,
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  difficulty: "medium",
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};
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// stored on heap
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let tags = ["react", "closures"];
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function startTimer() {
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  // used by the inner function, so kept on the heap
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  let seconds = 0;
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  return () => ++seconds;
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}
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let onTick = startTimer();

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Stack vs heap

Stack

Heap

questionId

402

isPremium

true

slug

ref

question

ref