CODEWITHSUNDEEP
javascriptfunction
Srujan
Srujan

Reputation: 93

How a function got passed to another function?

I came across a weird code which I didn't understood. It would be great if I get explanation. 

function forEach(array, action) {
  for (var i = 0; i < array.length; i++) {
    action(array[i]);
  }
}

var numbers = [1, 2, 3, 4, 5], sum = 0;
forEach(numbers, function (number) {
  sum += number;
});
console.log(sum);

How the function got passed as parameter ? and how the inner function is getting that number.

Upvotes: 2

Views: 80

Answers (2)

Rajesh
Rajesh

Reputation: 24945

In javascript, everything is an object, including function.

For simplicity, check this example:

var func = function(number){
  console.log(number);
}

var arr = [1,2,3,4,5];

function foreach(data, callback){
  for(var i = 0; i<data.length; i++){
    callback(data[i]);
  }
}

foreach(arr, func);

In JS, you can assign function to a variable. This is called as Function Expression. So it acts as a normal variable, just that its type is object.

This allows us to pass function as a parameter.

Now your second question, how the inner function is getting that number.

in your code, action(array[i]); is passing value as a parameter, so 

function (number) {
  sum += number;
}

here number is array[i].

Upvotes: 0

Madara&#39;s Ghost
Madara&#39;s Ghost

Reputation: 174997

In JavaScript, functions are what's called first class citizens (that's not necessarily true for other languages), that means that functions can be treated as data, you can pass functions as arguments, return functions from other functions, and create functions and assign them to variables.

Here's a very simple example:

function callWithoutParams(fn) {
  return fn(); // fn is the passed function.
}

let rand = callWithoutParams(Math.random); // Passing Math.random as a function.
callWithoutParams(() => console.log(42));  // Passing anonymous function

Array.prototype.forEach() accepts a function as an argument, and runs it on each element in the array. So a simpler example is:

[1,2,3,4,5].forEach((element) => console.log(element)); // outputs 1-5 in order.

Upvotes: 6

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