How does this simple Javascript program work?

Question

I was gearing up for Javascript, reading the tutorials at W3Schools and came across this code:

function sortNumber(a, b)
{
return a - b;
}

var n = ["10", "5", "40", "25", "100", "1"];
document.write(n.sort(sortNumber));

It sorts the elements in the Array, pretty simple. But how can we pass sortNumber ( a function name) as a parameter to the sort function?

The original example

Answer 1

You're actually passing a reference to the function itself, not its name. If you were passing its name, you would enclose it in quotes like this: "sortNumber".

sort() (as the documentation says) takes an optional argument which specifies the sorting function.

Answer 2

But how can we pass sortNumber ( a function name) as a parameter to the sort function?

Functions are first-class objects in JS and can be passed around as parameters or variables. Are you clear how the sort itself is working?

Answer 3

If you look at the documentation for the sort function within Javascript, the parameter that it accepts, which is optional is a function that can be used to determine the sort.

The function returns a positive, negative or zero value allowing the sort function to determine where everything needs to go.

Answer 4

The answer is available on W3Schools as well : http://www.w3schools.com/jsref/jsref_sort.asp

the Array sort() method takes one parameter which is a function handler.

Answer 5

A variable in javascript can contain a anytype of value, that includes functions. If you use the name of the function without the parenthesis, your referencing the function, and not actually calling it.

Answer 6

Functions are first class citizens in javascript.

Sort can take an optional paramater, a function that returns a -1, 0 or 1(whether a greater, equal to or lessthan b)

Using a-b returns a positive, zero or negative number, to do this.

Answer 7

Surprisingly, JavaScript has its roots in a language called Scheme.

Scheme allows certain functions (called 'lambda functions') to be passed around as though they were a variable.

JavaScript handles functions in much the same way scheme did. (Some people say that "functions are first-class-citizens in JavaScript.")

For example, you can write:

// Assign a function to foo
var foo = function () { alert('bar'); };

// Call foo like a function
foo();

The result would be that the message 'bar' is shown.

The classic example of lambdas is the "Adder" example:

adder = function (x) {
    return function (y) {
        x + y
    }
};
add5 = adder(5);
add5(1); // == 6

Hope this helps.