SetTimeout recursive with arrow function

user_14326456 0
Viewed 260

I would like to know if there is a way to use setTimeout recursive implements with an arrow function, in order to use this (refers to my class attribute for example) inside. Indeed, this = undefined when i declare my setTimeout with a normal function

I got :

public currentIndex: number = 0;

setTimeout(function run(){
    this.currentIndex++;
    console.log(this.currentIndex); // returns undefined
    setTimeout(run, 1000);
}, 1000)

Instead of :

setTimeout(() => {
    this.currentIndex++;
    console.log(this.currentIndex) // returns currentIndex value
    setTimeout( ?? , 1000) // What should i put instead of '??' ?
}, 1000)
angular typescript javascript
4 Answers

You could bind this first and then use this function for all calls.

function run(reference) {
    this.currentIndex++;
    console.log(this.currentIndex); // returns undefined
    setTimeout(reference, 1000, reference);
}

const runThis = run.bind(thisReference);

setTimeout(runThis, 1000, runThis);
user_14476750

Its because arrow function does not create new context inside arrow function body but normal function does. So this in arrow function refers to parent scope context but this in normal function refers to its own context.

user_127040300

This will create setTimeouts recursively

let currentIndex = 0;

const run = () => {
    setTimeout(() => {
        currentIndex++;
        console.log(currentIndex);
        run();
    }, 1000);
}

run();

but better approach may be (I don't know your use case, so it is just maybe) to use setInterval()

let currentIndex = 0;

const interval = setInterval(() => {
    currentIndex++;
    console.log(currentIndex);

    // stop interval
    if (currentIndex >= 10) {
        clearInterval(interval);
    }
}, 1000);

user_90742720

Probably the easiest way is to extract the arrow function into its own variable:

const run = () => {
    this.currentIndex++;
    console.log(this.currentIndex);
    setTimeout(run, 1000);
};
setTimeout(run, 1000);

Though in this particular example you could simplify it even more using setInterval instead of setTimeout, avoiding the second setTimeout call entirely.

user_93165540
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