How to use setTimeout inside a for loop in JavaScript

In this tutorial, we are going to learn about the how can we use setTimeout method inside a for loop in JavaScript with the help of examples.

for loop

Let’s see what happens when we add a setTimeout method inside a for loop.

for (var i=0;i<5;i++){
    setTimeout(function(){
        console.log(i);
    }, 1000);
}

//---> Output  5,5,5,5,5

After 1000 milliseconds we will see a 5 is logged inside browser console but we need 0,1,2,3,4 this happens because of JavaScript is executing the code in synchronous fashion(it means one line after another) but setTimeout method is asynchronous so that JavaScript runs the asynchronous code once the synchronous code execution is completed.

At the time of synchronous code (for loop) execution is completed the variable i value is 5 so that we can see 5 inside our console.

Using let keyword

The problem can be solved by using an es6 let keyword because it creates a new variable on each iteration but var keyword is using the same variable throughout the for loop execution.

for (let i=0;i<5;i++){
    setTimeout(function(){
        console.log(i);
    }, 1000);
}

// -- > output  0,1,2,3,4

Using IIFE function

If you don’t like using es6 let keyword, we can also solve the problem by using immediately invoked function expression (IIFE).

Example:

for (var i = 0; i < 5; i++) {
  (function(val) {     //val is parameter
    setTimeout(function() {
      console.log(val);
    }, 1000);
  })(i); // i is argument
}

// -- > output  0,1,2,3,4

In the above code, on each iteration the function is invoked by itself a var i is passed an argument to the function so that we can see the expected output because on each iteration the variable i is holding a different value.

Using functions

for (var i = 0; i < 5; i++) {
  function timeout(val) {
    setTimeout(function() {
      console.log(val);
    }, 1000);
  }
  timeout(i);
}