example how and what for to use anonymous functions

I wrote some short introduction to anonymous function here, some time ago. I wrote some simple example, but no example for real life situation. Today I decided to write down an example which could be very useful on technical interview.

This is a very simple code example. We have a loop and inside this loop we create an array of functions. Every of functions logs i on the console. Then in the end we are calling the first function from the array.

var i = 0;
var funcArray = [];
for (; i < 20; i += 2) {
  funcArray.push(function() {
    console.log('i inside function', i);
  };
});

funcArray[0]();

What we want to achieve here is, to have an array of functions, to use inside of function a value of i. We cannot use index of function in array, as i is incremented by 2. This implementation looks like the easiest solution for our need.

So what would display on console as a result of this line funcArray[0]();? We expect it to be 0, but unfortunately it is... 20... WHY??

The answer is very easy - the scope. The thing which makes JavaScript harder to learn. But is very logic. Scope exists on function level (ES5) or on block level (ES6+). In this case inside of loop the function which uses scope variable i is created. But the function is called outside the loop, where i has a value 20 (the first value, which didn't meet the requirement, so there was no more incrementation of i).

So it doesn't matter which index would be called, the i outside of loop is always equal the first value, which didn't pass the condiition. It means we have to change the way how we create the scope for our function. And here the anonymous function will come with help.

What should be done, then? We have to create immediately invoked anonymous function, which will create the scope and will return the function which will be assigned to array element. As a parameter to anonymous function i will be passed to make it available inside the scope.

funcArray.push(( // immediately invoked function
// anonymous function, which gets i from scope as index
  function (index) {
    return function() { // returns a our function
      console.log('index inside function', index);
    }
  }
)(i)); // i passed to immediately invoked function