July 21, 2011

How to detect a JavaScript array

"Piece of cafe" I bet you're thinking, oh I wish it were that easy.

But fear not, JavaScript does have the typeof operator (returns a string) which is very useful for type-checking:
var str = 'hello',
    num = 64,
    bool = false,
    func = function () {},

typeof str; // "string"
typeof num; // "number"
typeof bool; // "boolean"
typeof func; // "function"
typeof undef; // "undefined"
Like shown in the previous code, it works fine for most things. However, it's a little flawed:
var n = null,
    str = new String('hello'), // any wrapper function or custom constructor
    arr = [1, 2, 3];

typeof n; // "object", which is wrong
typeof str; // "object", which is correct but not very useful.
typeof arr; // "object", which is correct but not very useful.

constructor property
Every created object has a constructor property that points to the function that created the object's prototype. You can leverage that reference to do some accurate detections:
var str = new String('hello'),
    arr = [1, 2, 3],
    Person = function (name) {
        this.name = name;

obj = new Person('John');

obj.constructor === Person; // true
str.constructor === String; // true
arr.constructor === Array; // true
isArray method
ECMAScript 5 defines a new method for the Array function called isArray(). If it's not implemented by the browser you can define it like this:
if (!Array.hasOwnProperty('isArray')) {
    Array.isArray = function (value) {
        return Object.prototype.toString.call(value) === '[object Array]';
Invoking the toString() method of Object, gets us a string representation of the object, for arrays it's [object Array]:
var arr1 = [1, 2, 3],
    arr2 = new Array(1, 2, 3);

Array.isArray(arr1); // true
Array.isArray(arr2); // true
Array.isArray(new Number('12')); // false
Array.isArray({}); // false

typeof operator (MDC)
constructor property (MDC)

July 9, 2011

The this keyword in JavaScript

What does this mean?
Short answer, it depends.

In general terms, you could say that the this keyword, in JavaScript, refers to the function's context, how it's invoked. There are four ways in which a function can be invoked: method form, function form, constructor form and apply form.

Method form
If a function is invoked in the method form, this will be bound to the object that owns the function:
var myObj = {
    name: 'Chuck',
    sayName: function () {
        return 'Hello ' + this.name;

myObj.sayName(); // this refers to myObj, returns 'Hello Chuck'

Function form
This means taking a function value and calling it. In ECMAScript 3, this would be bound to the global object:
var myFunc = function () {
    return this.location.href;

myFunc(); // this refers to window (in browsers), returns the current URL
In ECMAScript 5 Strict Mode (almost fully supported), this was changed so that it would refer to undefined instead:
var myFunc = function () {
    'use strict';
    return this.location.href;

myFunc(); // error, this is undefined

Constructor form
A constructor is meant to be called using the new keyword, in this form, this will refer to the object being produced:
var MyConstructor = function (name) {
    this.name = name;
    this.sayName = function () {
        return 'Hello ' + this.name;

var myObj = new MyConstructor('Chuck');
myObj.name; // 'Chuck'
myObj.sayName(); // 'Hello Chuck'

Apply form
Functions are objects and, as most objects, they have methods. This form refers to using the function's methods apply() or call() to call a function. Both methods take whatever first argument you pass and bound this to it. They differ in the way they provide the arguments to the function being invoked.
The apply() method takes an array of arguments:
var myObj1 = {
    name: 'Chuck'
var myObj2 = {
    name: 'Charles'
var sayWelcome = function (location) {
    return 'Hello ' + this.name + '. Welcome to ' + location;

sayWelcome.apply(myObj1, ['New York']); // 'Hello Chuck. Welcome to New York'
sayWelcome.apply(myObj2, ['Toronto']); // 'Hello Charles. Welcome to Toronto'
The call() method takes a list of arguments separated by commas:
sayWelcome.call(myObj1, 'New York'); // 'Hello Chuck. Welcome to New York'
sayWelcome.call(myObj2, 'Toronto'); // 'Hello Charles. Welcome to Toronto'

The value of this depends on the calling form:
  • Method form: it's bound to the object containing the method.
  • Function form: it's bound to either the global object (ECMAScript 3), or to undefined in (ECMAScript 5 Strict Mode).
  • Constructor form: it's bound to the new object being constructed.
  • Apply form: it's bound to the first argument passed to apply() or call().

Crockford on JavaScript - Act III: Function the Ultimate