how to use function(1)(2) in javascript? and how does it work?

Question

I understand calling function(1) but not function(1)(2), how does it work?

also possible for function(1)(2)(3)(4) too?

Answer 1

In this case you are supposing that function(1) returns a function, than you are calling this new, anonymous function with an argument of 2.

See this example:

function sum(a) {
    return function(b) {
        return a+b;
    }
}

// Usage:
window.alert(sum(5)(3));         // shows 8

var add2 = sum(2);
window.alert(add2(5));           // shows 7
window.alert(typeof(add2));      // shows 'function'

Here we create a function sum that takes one argument. Inside the function sum , we create an anonymous function that takes another argument. This anonymous function is returned as the result of executing sum .

Note that this anonymous function is a great example of what we call closure . A closure is a function that keeps the context in which it was created. In this case, it will keep the value of the variable a inside it, as did the example function add2 . If we create many closures, they are independent as you can see:

var add3 = sum(3);
var add4 = sum(4);

window.alert(add3(3)); // shows 6
window.alert(add4(3)); // shows 7

Furthermore, they won't get "confused" if you have similarly named local variables:

var a = "Hello, world";

function multiply(a) {
    return function(b) {
        return a * b;
    }
}

window.alert(multiply(6)(7)); // shows 42

var twoTimes = multiply(2);
window.alert(typeof(twoTimes));
window.alert(twoTimes(5));

So, after a call to sum(2) or multiply(2) the result is not a number, nor a string, but is a function . This is a characteristic of functional languages -- languages in which functions can be passed as parameters and returned as results of other functions.

Answer 2

You have a function that returns a function:

function f(n) {
  return function(x) {
    return n + x;
  };
}

When you call f(1) you get a reference to a function back. You can either store the reference in a variable and call it:

var fx = f(1);
var result = fx(2);

Or you can call it directly:

var result = f(1)(2);

To get a function that returns a function that returns a function that returns a function, you just have to repeat the process:

function f(n) {
  return function(x) {
    return function(y) {
      return function(z) {
        return n + x + y + z;
      }
    }
  };
}

Answer 3

If your function returns a function, you can call that too.

x = f(1)(2)

is equivalent to:

f2 = f(1)
x = f2(2)

Answer 4

The parenthesis indicate invocation of a function (you "call" it). If you have

<anything>()

It means that the value of anything is a callable value. Imagine the following function:

function add(n1) {
    return function add_second(n2) { 
        return n1+n2
    }
}

You can then invoke it as add(1)(2) which would equal 3. You can naturally extend this as much as you want.