How can I count how many times a function argument has been used in JS?

Question

I need to construct a function, which takes a black-box one-argument function as an argument. And I need to know, how many times the black-box function uses its argument

Example signature:

function argCounter(fn) {
  return (arg) => {
    let argCalls = 0

    ...

    fn(arg)

    return argCalls
  }
}

Example use:

const fn1 = argCounter(obj => {
  return obj
}

fn1() // 1

const fn2 = argCounter(obj => {
  obj; obj; obj;
  return obj
}

fn2() // 4

I know about JS Proxy, but it only gives me the ability to count how many times an object's methods or attributes have been called. Unfortunately, I couldn't determine, how to use JS Proxy to count, how many times the object itself has been used.

A black box is a black box, you can't look inside. If it's not completely black, please tell us what you do know about the function. What do you need this for? — Bergi
– Bergi, Commented Oct 19, 2024 at 21:21
possibly Proxy? just a usage cannot be tracked, maybe get values returned via getObj? — user1514042
– user1514042, Commented Oct 19, 2024 at 21:47
How do you want to handle the case where inside of a blackbox function the single argument gets assigned to another constant where the latter than is going to be used instead ... e,g. ... const fn2 = argCounter(obj => { const ref = obj; ref; ref; ref; return obj; });? The only way anyhow was to stringify the blackbox-function and pray that whatever is the result can be parsed without error. Thus one actually does not want to go that path. — Peter Seliger
– Peter Seliger, Commented Oct 19, 2024 at 23:07

Bergi · Accepted Answer · 2024-10-19 21:22:07Z

2

This is not possible. There is no hook that is called when a value is referred to.

Notice that this also is subject to optimisation by the engine. The example you gave (obj => { obj; obj; obj; return obj }) is totally equivalent to obj => obj in behaviour.

answered Oct 19, 2024 at 21:22

Bergi

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