Question 292972 in Javascript Question posted in
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Javascript – How to capture the values that were present at the time of calling asynchronous functions

let x = "value before setTimeout";

function cb() {
    console.log(currentValue); 
}

setTimeout(cb, 3000);

x = "updated value";

How to print "value before setTimoeout" with the help of callback function. How to capture the old value and use it?

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3

Answers


  1. 0

    Store it as a temporary coefficient:

    let a = 20

function log(a) {
  console.log('a is: ' + a)
}

setTimeout(log, 1000, a)

a = 43
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  2. 0

    You can wrap your callback function inside another one that will keep the currentValue in its closure so you can access it later.
    The main difference is that your callback function has to be called once now (cb(x)) in order to bind that value.

    let x = "value before setTimeout";

function cb(currentValue) {
  return function() {
    console.log(currentValue);
    console.log(x);
  }
}

setTimeout(cb(x), 3000);

x = "updated value";
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  3. 0 
    let x = "value before setTimeout";

function cb(val) {
  console.log(val);
}

setTimeout(cb, 3000, x);

x = "updated value";

    By passing x as an argument to the callback function, you can capture its value at the time setTimeout is set, before it gets updated. And then print that captured value inside the callback.

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