Anonymous Function

Supporting anonymous function means we treate function as first class citizen. So you can pass a function as a parameter, or returned as a value. JavaScript is actually a functional programming language. It supports first class function and allow us to define an anonymous function easily.

Define Anonymous Function

    return x*x;
}// => SyntaxError: function statement requires a name

Why we got a SyntaxError here? Because we need a function expression to create an anonymous function.

Expression should returns a value:

var a = new Array()  // new Array() is an expression, and the whole line is called a statement.

But why the error says: function statement requires a name. In JavaScript, there is another way to declare a function, that is using function statement. A statement requires a name. The code above is treated as function statement, because there is nothing to return. Therefore, we got the error.

If we assign function (x){return x*x} to a variable, or pass it as a parameter, we will not get any SyntaxError, because function (x){return x*x} is only a expression under this circumstance. For instance:

var squareA = function(x){
    return x*x;

However, a tricky thing is that squareA becomes a named function.

console.log(squareA)  // => function squareA()

Even squareA becomes a named function, but the definition process is different from code below:

function squareB(x){
    return x*x;
} // => undefined

squareB directly defines a function using function statement that obviously has no return. While, squreA defines an anonymous function using function expression, and assigns the function returned from function expression to a variable named squareA. So, a expression returns a value:

console.log(function(x){ return x*x});
// => undefined
// => function ()

The undefined is the return value for console.log(), and function() is the return value from function expression that defines an anonymous function.

Although squareA looks like a named function, but while you construct an Object from it, that will be a bit funny:

new squareA() // => ""
new squareB() // => "squareB"

Use Anonymous Function

Function is first class, that means we can assign a function to a variable:

var square = function(x) {return x*x}

We can pass the anonymous function as a parameter as well. just like we passed an anonymous function to console.log:

 console.log(function(x){return x*x})

And, we can return a function just like return a value:

function multiply(x){
    return function(y){
        return x*y;

multiply(1)(2) // => 2