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jQuery is built on top of JavaScript, a rich and expressive language in its own right. This section covers the basic concepts of JavaScript, as well as some frequent pitfalls for people who have not used JavaScript before. While it will be of particular value to people with no programming experience, even people who have used other programming languages may benefit from learning about some of the peculiarities of JavaScript.

Here's a simple JavaScript program that adds a message to the page:

// create a function that will greet a person,
// and assign the function to the `greet` variable
var greet = function( person, message ) {
  var greeting = 'Hello, ' + person + '!';
  log( greeting + ' ' + message );
};

// use the function to greet Jory, passing in her
// name and the message we want to use
greet( 'Jory', 'Welcome to JavaScript' );

// use the function to greet Rebecca, passing in her
// name and a different message
greet( 'Rebecca', 'Thanks for joining us' );
In the above example, we use a function called log. This is a helper function that is defined by the JavaScript that powers this site — it is not a built-in JavaScript function. You can use log in the built-in editor for this site, but you'll need to use console.log in your normal code, and then view the output in your browser's console.

You can try running this program by clicking the and it will copy the code to the built-in editor automatically. Once it's there, you can click the button to execute it; you can also make changes to it. Press the button to return to the original code.

It's OK if you don't understand everything that's going on in this program; in the rest of this section, we'll talk about variables, functions, and several other building blocks of JavaScript.

A comment about comments

Comments can be a useful tool for explaining what's happening in a piece of code. The contents of a comment will have no effect whatsoever on how your code runs. Some people like to heavily comment their code, while others prefer to include comments only to explain things that may not be immediately obvious.

JavaScript allows for single-line and multi-line comments. A single-line comment begins with //; a multi-line comment begins with /* and ends with */.

// This is a single line comment
var foo;

/*
  This is a multi-line comment. It can go on
  for several lines, like this.
 */

You'll also sometimes see multi-line comments used for inline comments:

function cornify( unicorns /* integer */, rainbows /* integer */ ) {

}

The building blocks of JavaScript

Variables

Variables are the way that we store values so we can use them later. Variables can contain any value — text, numbers, data (such as "arrays" and "objects"), even code (in the form of "functions"). We declare a variable using a var statement:

var myName = 'Rebecca';

Variable names can be just about anything, as long as they don't begin with a number and don't include a hyphen.

You can define one variable per statement:

var a = 1;
var b = 'two';

You can also define multiple variables in a single statement, by separating them with commas:

var a = 1,
    b = 'two';

Once you have assigned a value to a variable, you can use that variable whenever you want to get access to the value you stored.

log( myName ); // logs 'Rebecca'

Variables are a key ingredient in adhering to the Don't Repeat Yourself philosophy. If you need to use a value more than once, you probably want to store it in a variable.

We'll talk more about variables in the next section, Functions.

Functions

Functions are a fundamental building block of JavaScript programs; they provide a way that we can put small pieces of functionality in a neatly-wrapped package. For example, consider a function that adds two numbers:

function(a, b) {
  return a + b;
}

This function takes two parameters, a and b. It adds them together, and returns the result.

This is a valid bit of JavaScript, but as written, there's no way for us to call our function if we actually want to add two things. We can solve this by assigning our function to a variable:

var addTwoNumbers = function(a, b) {
  return a + b;
};

What we've done here is take a function expression and assign it to a variable. Now that we've done this, we can call our function by using the name of the variable, passing 1 and 2 as arguments into the function:

log( addTwoNumbers(1, 2) ); // logs 3

We could also use a function declaration to give our function a name:

function addTwoNumbers(a, b) {
  return a + b;
}

This lets us call our function just like we did before, but this approach should be used with caution, as explained in this post.

Bottom line: naming functions using the function declaration approach can have unexpected results if you don't fully understand a feature of JavaScript known as hoisting. The details of hoisting are beyond the scope of this guide, but for now we'll stick to assigning function expressions to variables.

Functions and variable scope

Variables that are declared inside a function with a var statement are only available inside of the function; this is generally desirable, and all variables should be declared with a var statement unless they are intended to be global — that is, available anywhere in your code. This usually isn't what you want, unless you want it to be possible for other code to change the value of the variable.

What does it mean that a variable is only accessible inside of a function? Give the following code a try:

```<span class="caution">caution</span> broken code
var myFunction = function() {
  var foo = 'bar';
};

myFunction();
log( typeof foo ); // logs undefined!

In the example above, if we'd tried to actually use foo outside of the function — rather than just checking its type — our browser would have reported an error, and any code after the line where we used foo would not run.

The next example shows how we can have two variables with the same name, as long as each variable exists in a separate scope. In this example, we declare the variable foo and assign it the value 'qux'; then, inside a function, we declare another variable named foo, and assign it the value 'bar'. Notice how, outside of the function, the variable foo does not change, even when we create the variable foo inside the function.

var foo = 'qux';
var myFunction = function() {
  var foo = 'bar';
};

log( foo ); // logs 'qux'
myFunction();
log( foo ); // still logs 'qux'

Despite the fact that both variables have the same name, JavaScript considers the two variables as completely separate entities. This is just one of many reasons that it's important to give your variables meaningful names.

Variable scope is one of the fundamental concepts of JavaScript, and one of the concepts that people struggle with often. It's important to remember that:

  • you should almost always declare variables with a var statement
  • variables declared inside of a function using a var statement are not available outside of that function
  • variables declared without a var statement are always global

Beware that variables that are not declared with the var keyword are implicitly global! In the following example, the variable a is available outside the function because it wasn't declared with the var keyword — this is generally undesirable.

```<span class="caution">caution</span> unsafe code
function test() {
  a = 1;
}

test();

log( a ); // logs 1

Objects

As it turns out, most everything we work with in JavaScript is an object — in fact, there are only five kinds of values that are not objects:

  • strings (text)
  • booleans (true/false)
  • numbers
  • undefined
  • null

These values are called primitives, but even some of these values can be treated as though they were objects — more on that in a minute. But what's an object? Let's look at an example of a simple object:

var person = {
  firstName : 'Boaz',
  lastName : 'Sender'
};

The object we've created here has two properties: firstName and lastName. We've created it using the "object literal syntax" — that is, by putting a set of key-value pairs in { }. Note that, for each pair, there is a colon between the key and the value, and there is a comma between each pair. Note also that there is not a comma after the last key-value pair — if you accidentally include a comma after the last pair, you will get errors in older browsers.

Accessing properties

We've stored our object in a variable named person, which makes it easy to access its properties, using either dot notation or bracket notation.

var person = {
  firstName : 'Boaz',
  lastName : 'Sender'
};

log( 'First name: ' + person.firstName );     // dot notation
log( 'Last name: ' + person[ 'lastName' ] );  // bracket notation

You'll notice that with bracket notation, we used a quoted string for the property name we were after; with dot notation, we just used the literal property name, without quotes. Bracket notation is a useful approach if, say, the name of the property you're after is stored in a variable:

var person = {
  firstName : 'Boaz',
  lastName : 'Sender'
};

var prop = 'lastName';

log( 'Last name: ' + person[ prop ] );

Once we've created an object, we can modify its properties.

var person = {
  firstName : 'Boaz',
  lastName : 'Sender'
};

person.firstName = 'Ben';
person.lastName = 'Alman';

log( 'First name: ' + person.firstName );
log( 'Last name: ' + person.lastName );

This aspect of JavaScript is both blessing and curse. It means that objects are incredibly flexible, but it also means that there's no "privacy." Any code can easily overwrite the value of a property of any object to which it has access — another reason why it's important to keep variables out of the global scope unless it's OK for other code to modify them.

Object methods

Methods of an object are just properties whose values are functions. Let's add a .greet() method to our person object:

var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function(name) {
    log( 'Hi, ' + name );
  }
};

person.greet( person.firstName );

The .greet() method in the example above received a string, name, as its argument. When we called the method, though, we simply passed in the value of the firstName property of the person object. If we wanted a super-flexible .greet() method that could greet anyone, this might be what we want. But it probably makes more sense that the .greet() method will greet the particular person.

The meaning of this

Inside of a method — indeed, inside of any function — there is a special keyword available to us: this. It refers to the object that is the context in which the function was called.

When we call person.greet(), the context object is person itself. This means that we can use this to access a property of the person object from directly within the .greet() method.

The meaning of this can be incredibly perplexing to new JavaScript developers, and you should take comfort in knowing that jQuery largely makes it so that you don't need to understand it for a while. However, no discussion of objects and methods is complete without talking about this at least a little. In short, if this section is confusing, feel free to skip to Objects in jQuery, and come back to it when you're ready.

Let's look at how we could use this in our method.

var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function() {
    log( 'Hi, ' + this.firstName );
  }
};

person.greet();

Not so confusing so far, right? The confusion arises because the meaning of this can change — as mentioned above, it depends on the context in which the function was called! Consider the following code:

```<span class="caution">caution</span> broken code
var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function() {
    log( 'Hi, ' + this.firstName );
  }
};

var sayIt = person.greet; // store the method in a variable

sayIt(); // logs 'Hi, undefined' -- uh-oh

When we store the .greet() method in a variable sayIt and then call sayIt(), the context object changes to the global window object, not the person object. Since the window object doesn't have a property firstName, we get undefined when we try to access it.

What's a developer to do? First, be aware that storing object methods in variables can have unintended consequences for the meaning of this. Second, be aware that you can force the meaning of this to be what you want it to be, using the .call() or .apply() method on the function itself.

var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function() {
    log( 'Hi, ' + this.firstName );
  }
};

var sayIt = person.greet;

sayIt.call( person );

Both .call() and the very similar .apply() method also let us pass arguments to the function we're invoking. Imagine if our greet method took some arguments; we could pass those arguments using .call() like this:

var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function(greeting, punctuation) {
    log( greeting + ', ' + this.firstName + punctuation );
  }
};

var sayIt = person.greet;

sayIt.call( person, 'Hello', '!!1!!1' );

We could do the same thing using .apply(), but we'd pass the arguments within a single array instead of as separate arguments:

var person = {
  firstName : 'Boaz',
  lastName : 'Sender',
  greet : function(greeting, punctuation) {
    log( greeting + ', ' + this.firstName + punctuation );
  }
};

var sayIt = person.greet;

sayIt.apply( person, [ 'Hello', '!!1!!1' ] );
For more details about .call() and .apply(), see the MDN documentation on .call() and .apply().

Objects in jQuery

We've barely scratched the surface of objects, but you now know the basics you'll need to know to work with objects as you learn jQuery. In basic jQuery, you'll mainly use objects to provide configuration options. For example, you can provide an object to change several CSS properties on an element at once:

$('#main').css({
  color: 'red',
  border: '1px solid blue'
});

As far as this, jQuery tends to take control over its meaning. In the case of event handlers, this will refer to the element to which you bound the handler; in the case of iterating over a set of elements in a selection, this will refer to the current element in the iteration. You shouldn't need to worry about understanding this too much during your initial learning — it's just a good thing to keep in mind as your learning continues.

Further reading

The MDN JavaScript Guide goes into detail about topics such as object prototypes, constructor functions, and deleting object properties.

Arrays

Arrays are a type of object used to store lists of values. They are a handy way to store a set of related items of the same type (such as strings), though in reality, an array can include multiple types of items, including other arrays.

The preferred way to create an array is to use the array literal notation:

var myArray = [ 'a', 'b', 'c' ];
You will sometimes see code that creates an array using the new Array('a', 'b', 'c') construct. This is generally frowned upon among JavaScript developers; it provides no advantages over the literal construct, and has some disadvantages, such as the fact that new Array(3) will create an array with three undefined elements, rather than the array [ 3 ].

You can access properties of arrays (sometimes called elements) with the same bracket notation we used for objects. Each element is automatically given a name based on its position in the array. Be careful, though: the numbers start at zero! Lets look at an example array with three elements:

```<span class="caution">caution</span> antipattern
var myArray = [ 'a', 'b', 'c' ];
var firstItem = myArray[ "0" ]; // access the first item

When retrieving array elements, it's usually much more convenient to use numbers to specify the index of the element you're after:

var myArray = [ 'a', 'b', 'c' ];
var firstItem = myArray[ 0 ];

var secondItem = myArray[ 1 ]; // access the item at index 1
log( secondItem ); // logs 'b'

We can determine how many items are in an array by accessing an array's length property:

var myArray = [ 'a', 'b', 'c' ];
var len = myArray.length;
log( len ); // logs 3

for Loops: Iterating over Arrays

Since we know how to get the length of an array, and we know that an array's first item is at index 0, we can iterate over the items in an array using a for loop:

var myArray = [ 'a', 'b', 'c' ];
var i;
var len = myArray.length;

// we'll use the variable i as our index; it starts at 0,
// and we increment it by 1 (using i++) until i is no longer
// less than the length of the array
for (i = 0; i < len; i = i + 1) {
  log( 'item at index ' + i + ' is ' + myArray[ i ] );
}

There is a whole lot more that you can do with arrays; for a complete reference, see the Mozilla Developer Network's Array documentation.

Logic and Truthiness

JavaScript provides if and else, as well as the ternary operator, to allow us to do certain things only when certain conditions are met. JavaScript determines whether a condition is met by evaluating a value or an expression for its "truthiness." Because JavaScript is a dynamically typed language, we can use any value or combination of values; however, the rules by which JavaScript determines whether a given value or expression is true or false can be confusing.

Here's an example of a simple if statement in JavaScript. It evaluates the truthiness of the number 1; because 1 is truthy, the code inside the block — delineated by { and } — will run.

if ( 1 ) {
  // this code will run!
  log( '1 is truthy' );
}

As it turns out, most values in JavaScript are truthy — in fact, there are only five values in JavaScript that are falsy:

  • undefined (the default value of declared variables that are not assigned a value)
  • null
  • NaN ("not a number")
  • 0 (the number zero)
  • '' (an empty string)

When we want to test whether a value is "falsy," we can use the ! operator:

var a = '';

if ( !a ) {
  // this code will run if a is falsy
  log( 'a was falsy' );
}

The value NaN is a special case. Values that are NaN will evaluate to false in a simple conditional expression:

var notANumber = 'four' - 'five';

if ( !notANumber ) {
  // this code will run
  log( '!notANumber was truthy' );
}

However, if we compare the value NaN to false, we get back a falsy value:

var notANumber = 'four' - 'five';

if ( notANumber == false ) {
  // this code will not run!
  log( 'notANumber was falsy' );
} else {
  // this code will run
  log( 'notANumber was truthy' );
}

It's important to remember that all other values aside from the five values listed above are truthy. This includes empty arrays, empty objects, all non-empty strings (including the string '0'), and all numbers other than 0.

It is possible to write single-line if and else statements without using curly braces. This practice is discouraged, as it can make code difficult to read and maintain. It is mentioned here simply because you may encounter it in others' code.

Logical Operators

Logical operators allow you to evaluate operands using AND (&&) and OR (||) operations.

var foo = 1;
var bar = 0;
var baz = 2;

foo || bar;     // returns 1, which is truthy
bar || foo;     // returns 1, which is truthy

foo && bar;     // returns 0, which is falsy
foo && baz;     // returns 2, which is truthy

In the case of the || operator, the value returned is the first value that proves the statement truthy, or the last value. In the case of the && operator, the value returned is the first value that proves the statement falsy, or the last value.

You'll sometimes see logical operators used as a clever way to control code execution:

foo && bar();   // runs bar() only if foo is truthy

var bar = baz || createBar();  // use baz as value for bar unless
                               // baz is falsy; in that case, create bar

This style is elegant and pleasantly terse, but it can be difficult to read, especially for beginners. You should be aware of what it means if you come across it in code, but you may want to refrain from using this style at first, and use traditional if and else statements for clarity.

The Ternary Operator

Often, you'll want to set a variable's value differently depending on whether a certain condition is true or false. Of course, you could achieve this with an if and else statement:

```<span class="caution">caution</span> antipattern
var propertyName;

if (dim === 'width') {
  propertyName = 'clientWidth';
} else {
  propertyName = 'clientHeight';
}

However, the ternary operator provides a much more succinct way of conditionally setting the value of a variable:

var propertyName = ( dim === 'width' ) ? 'clientWidth' : 'clientHeight';

The statement before the ? is evaluated for its truthiness. If it is truthy, the first value ('clientWidth') is used as the value of the propertyName variable; otherwise, the second value ('clientHeight') is used.

JavaScript gotchas

In addition to variable scope and truthiness, there are many other gotchas in JavaScript. Let's take a look at a few of them.

Naming things

Valid names in JavaScript begin with a letter or certain symbols, followed by zero or more letters, digits, underscores, or symbols. Names may not begin with numbers, and may not include hyphens. Besides these rules, you can name your variables however you'd like! All of these names are valid:

  • a
  • a1
  • foo_bar
  • fooBarBaz
  • $fooBar
  • _foo
  • __foo__

There are some conventions that people use when naming things in JavaScript, but these are optional, and don't have any effect on how the code works:

  • Names that begin with _ are usually "private" (more on this later).
  • Names that begin with uppercase letters are usually "constructors," used to create new instances of objects (more on this later as well).
  • In code that uses jQuery, names that begin with $ usually refer to jQuery objects.

Reserved words

JavaScript reserves certain words for its own use; you should avoid using these words for the names of things.

abstract boolean break byte case catch char class const continue debugger
default delete do double else enum export extends false final finally float
for function goto if implements import in instanceof int interface long
native new null package private protected public return short static super
switch synchronized this throw throws transient true try typeof var
volatile void while with

If you must use one of these names as the name of an object property, you should quote the property name:

var myObject = {
  'class': 'tasty'
};

Operations on numbers and strings

Working with numbers in JavaScript can be a bit unpredictable, due to the way JavaScript represents numbers internally. For example, addition using decimals can have results that are close but not quite accurate:

log( 0.0001 + 0.0002 ); // 0.00030000000000000003

This won't come up often when working with jQuery, but it's an important issue to be aware of, especially if you're working with decimal numbers like currency. If you're doing mathematical operations that require precision, you should convert values to integers before working with them, and convert them back when you're done.

JavaScript is a loosely-typed language. If you try to do mathematical operations using values that are not numbers, JavaScript will not throw errors, and the result may not be what you'd expect.

log( 'a' + 2 );           // 'a2'
log( '4' + 3 );           // '43'
log( 'five' - '4' );      // NaN (not a number)
log( - '1' );             // -1
log( 1 + true );          // 2
log( 1 == true );         // true
log( 1 === true );        // false

Further reading

We've only scratched the surface of JavaScript. The Mozilla Developer Network is an excellent resource for information about the JavaScript language, particularly their JavaScript Guide. Specifically, you'll do well to read the following articles: