Solution for throttle decorator is incorrect. (Decorators and forwarding, call/apply)

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#3,613 opened on Nov 12, 2023 0 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 2/5
Estimated time: 1-3 hours
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Clearly specified
Activity status: Stale
Tech stack: javascript
Domain: documentation

Research direction

Start with the throttle decorator example in the linked Decorators and forwarding, call/apply tutorial section and reproduce the provided tight-loop snippet. Compare the current behavior with pull request #2844; done means the example no longer collapses the loop to only the first and last values.

Written by the indexing model from the issue text.

Description

Original code: https://javascript.info/call-apply-decorators#throttle-decorator

Here's a small code snippet to show where it doesn't work.

function f(a) { console.log(a) };

let g = throttle(f, 1000);

for(let i = 0; i < 1e8; i++) g(i);

Expected Output

1, 249204, 452039, ... , 9999999 (These are random increasing numbers)

Output

1, 9999999

Why does it fail?

function wrapper() {

    if (isThrottled) { // (2)
      savedArgs = arguments;
      savedThis = this;
      return;
    }
    isThrottled = true;

    func.apply(this, arguments); // (1)

    setTimeout(function() {
      isThrottled = false; // (3)
      if (savedArgs) {
        wrapper.apply(savedThis, savedArgs);
        savedArgs = savedThis = null;
      }
    }, ms);
  }

In above, isThrottled = false assignment is done inside setTimeout callback. However, only one callback is pushed into task queue and it isn't executed until stack is empty (for loop has to be completed).
isThrottled is always true => setTimeout isn't called => one callback (that was registered for initial false isThrottled) => cb executed at end and outputs last value => output: 1, 9999999.

Correct Solution: https://github.com/javascript-tutorial/en.javascript.info/pull/2844

This PR giving an alternative solution.

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