Functions contain blocks of code that need to be executed repeatedly. Functions can take zero or more arguments, and can optionally return a value.
Functions can be created in a variety of ways:
I prefer the named function expression method of setting a function's name, for some rather in-depth and technical reasons. You are likely to see both methods used in others' JavaScript code.
Example 2.34. A simple function
var greet = function(person, greeting) {
var text = greeting + ', ' + person;
console.log(text);
};
greet('Rebecca', 'Hello');
Example 2.35. A function that returns a value
var greet = function(person, greeting) {
var text = greeting + ', ' + person;
return text;
};
console.log(greet('Rebecca','hello'));
Example 2.36. A function that returns another function
var greet = function(person, greeting) {
var text = greeting + ', ' + person;
return function() { console.log(text); };
};
var greeting = greet('Rebecca', 'Hello');
greeting();
A common pattern in JavaScript is the self-executing anonymous function. This pattern creates a function expression and then immediately executes the function. This pattern is extremely useful for cases where you want to avoid polluting the global namespace with your code -- no variables declared inside of the function are visible outside of it.
Example 2.37. A self-executing anonymous function
(function(){
var foo = 'Hello world';
})();
console.log(foo); // undefined!
In JavaScript, functions are "first-class citizens" -- they can be assigned to variables or passed to other functions as arguments. Passing functions as arguments is an extremely common idiom in jQuery.
Example 2.38. Passing an anonymous function as an argument
var myFn = function(fn) {
var result = fn();
console.log(result);
};
myFn(function() { return 'hello world'; }); // logs 'hello world'
Example 2.39. Passing a named function as an argument
var myFn = function(fn) {
var result = fn();
console.log(result);
};
var myOtherFn = function() {
return 'hello world';
};
myFn(myOtherFn); // logs 'hello world'
Copyright Rebecca Murphey, released under the Creative Commons Attribution-Share Alike 3.0 United States license.