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.