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JavaScript

JavaScript Functions

Functions in JavaScript have really changed over the past ten years or so.

Function Declarations

Defining functions using declarations used to be the the usual way of using functions. Declared functions are not executed immediately. They are executed later when they are invoked.

Example

function myfunction(a, b) {
    return a * b;
}

myfunction(2, 3);

Anonymous Functions

You can define a function without a name. Such functions are known as anonymous functions.

Example

You can pass an anonymous function as a parameter in another function call.

setTimeout(function() {
  alert('hello');
}, 1000);

Function Expressions

You can also define an anonymous function using an expression, and you can store a function expression in a variable. You invoke the function by stating the variable name followed by a set of parenthesis and a semicolon.

Example

var sayhi = function() {
    alert('Hello World!');
}

sayhi();

But what problems do function expressions solve? Why were they added to the JavaScript language?

Closures

When you wrap an anonymous function definition in a set of parenthesis, you can invoke it by appending a set of parenthesis to it, followed by a semicolon.

Example

(function() {
    alert('foo');
})();

Lambdas

Lambdas provide a shorthand for creating anonymous functions.

Example

An anonymous function with one argument x that returns x + 1.

x => x + 1

Example

As above, but with a function body.

x => {return x + 1}

Example

A two-argument lambda function.

(x, y) => x + y

Example

Same as above, except with a function body.

(x, y) => {return x + y}

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