JavaScript Basics

Overview

JavaScript is 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.

If you’re interested in learning more about the JavaScript language, I highly recommend JavaScript: The Good Parts by Douglas Crockford.

Syntax Basics

Understanding statements, variable naming, whitespace, and other basic JavaScript syntax.

A simple variable declaration

var foo = 'hello world';

Whitespace has no meaning outside of quotation marks

var foo =         'hello world';

Parentheses indicate precedence

2 * 3 + 5;    // returns 11; multiplication happens first
2 * (3 + 5);  // returns 16; addition happens first

Tabs enhance readability, but have no special meaning

var foo = function() {
    console.log('hello');
};

Operators

Basic Operators

Basic operators allow you to manipulate values.

Concatenation

var foo = 'hello';

var bar = 'world';

console.log(foo + ' ' + bar); // 'hello world'

Multiplication and division

2 * 3;
2 / 3;

Incrementing and decrementing

var i = 1;

var j = ++i;  // pre-increment:  j equals 2; i equals 2
var k = i++;  // post-increment: k equals 2; i equals 3

Operations on Numbers & Strings

In JavaScript, numbers and strings will occasionally behave in ways you might not expect.

Addition vs. concatenation

var foo = 1;
var bar = '2';

console.log(foo + bar);  // 12. uh oh

Forcing a string to act as a number

var foo = 1;
var bar = '2';

// coerce the string to a number
console.log(foo + Number(bar));

The Number constructor, when called as a function (like above) will have the effect of casting its argument into a number. You could also use the unary plus operator, which does the same thing:

Forcing a string to act as a number (using the unary-plus operator)

console.log(foo + +bar);

Logical Operators

Logical operators allow you to evaluate a series of operands using AND and OR operations.

Logical AND and OR operators

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

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

foo && bar;   // returns 0, which is false
foo && baz;   // returns 2, which is true
baz && foo;   // returns 1, which is true

Though it may not be clear from the example, the || operator returns the value of the first truthy operand, or, in cases where neither operand is truthy, it’ll return the last of both operands. The && operator returns the value of the first false operand, or the value of the last operand if both operands are truthy.

Be sure to consult the section called “Truthy and Falsy Things” for more details on which values evaluate to true and which evaluate to false.

Note

You’ll sometimes see developers use these logical operators for flow control instead of using if statements. For example:

// do something with foo if foo is truthy
foo && doSomething(foo);

// set bar to baz if baz is truthy;
// otherwise, set it to the return
// value of createBar()
var bar = baz || createBar();

This style is quite elegant and pleasantly terse; that said, it can be really hard to read, especially for beginners. I bring it up here so you’ll recognize it in code you read, but I don’t recommend using it until you’re extremely comfortable with what it means and how you can expect it to behave.

Comparison Operators

Comparison operators allow you to test whether values are equivalent or whether values are identical.

Comparison operators

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

foo == bar;   // returns false
foo != bar;   // returns true
foo == baz;   // returns true; careful!

foo === baz;             // returns false
foo !== baz;             // returns true
foo === parseInt(baz);   // returns true

foo > bim;    // returns false
bim > baz;    // returns true
foo <= baz;   // returns true

Conditional Code

Sometimes you only want to run a block of code under certain conditions. Flow control — via if and else blocks — lets you run code only under certain conditions.

Flow control

var foo = true;
var bar = false;

if (bar) {
    // this code will never run
    console.log('hello!');
}

if (bar) {
    // this code won't run
} else {
    if (foo) {
        // this code will run
    } else {
        // this code would run if foo and bar were both false
    }
}

Note

While curly braces aren’t strictly required around single-line if statements, using them consistently, even when they aren’t strictly required, makes for vastly more readable code.

Be mindful not to define functions with the same name multiple times within separate if/else blocks, as doing so may not have the expected result.

Truthy and Falsy Things

In order to use flow control successfully, it’s important to understand which kinds of values are “truthy” and which kinds of values are “falsy.” Sometimes, values that seem like they should evaluate one way actually evaluate another.

Values that evaluate to true

'0';
'any string';
[];  // an empty array
{};  // an empty object
1;   // any non-zero number

Values that evaluate to false

0;
'';  // an empty string
NaN; // JavaScript's "not-a-number" variable
null;
undefined;  // be careful -- undefined can be redefined!

Conditional Variable Assignment with The Ternary Operator

Sometimes you want to set a variable to a value depending on some condition. You could use an if/else statement, but in many cases the ternary operator is more convenient. [Definition: The ternary operator tests a condition; if the condition is true, it returns a certain value, otherwise it returns a different value.]

The ternary operator

// set foo to 1 if bar is true;
// otherwise, set foo to 0
var foo = bar ? 1 : 0;

While the ternary operator can be used without assigning the return value to a variable, this is generally discouraged.

Switch Statements

Rather than using a series of if/else if/else blocks, sometimes it can be useful to use a switch statement instead. [Definition: Switch statements look at the value of a variable or expression, and run different blocks of code depending on the value.]

A switch statement

switch (foo) {

    case 'bar':
        alert('the value was bar -- yay!');
    break;

    case 'baz':
        alert('boo baz :(');
    break;

    default:
        alert('everything else is just ok');
    break;

}

Switch statements have somewhat fallen out of favor in JavaScript, because often the same behavior can be accomplished by creating an object that has more potential for reuse, testing, etc. For example:

var stuffToDo = {
    'bar' : function() {
        alert('the value was bar -- yay!');
    },

    'baz' : function() {
        alert('boo baz :(');
    },

    'default' : function() {
        alert('everything else is just ok');
    }
};

if (stuffToDo[foo]) {
    stuffToDo[foo]();
} else {
    stuffToDo['default']();
}

We’ll look at objects in greater depth later in this chapter.

Loops

Loops let you run a block of code a certain number of times.

Loops

// logs 'try 0', 'try 1', ..., 'try 4'
for (var i=0; i<5; i++) {
    console.log('try ' + i);
}

Note that in Loops even though we use the keyword var before the variable name i, this does not “scope” the variable i to the loop block. We’ll discuss scope in depth later in this chapter.

The for loop

A for loop is made up of four statements and has the following structure:

for ([initialisation]; [conditional]; [iteration])
 [loopBody]

The initialisation statement is executed only once, before the loop starts. It gives you an opportunity to prepare or declare any variables.

The conditional statement is executed before each iteration, and its return value decides whether or not the loop is to continue. If the conditional statement evaluates to a falsey value then the loop stops.

The iteration statement is executed at the end of each iteration and gives you an opportunity to change the state of important variables. Typically, this will involve incrementing or decrementing a counter and thus bringing the loop ever closer to its end.

The loopBody statement is what runs on every iteration. It can contain anything you want. You’ll typically have multiple statements that need to be executed and so will wrap them in a block ( {...}).

Here’s a typical for loop:

A typical for loop

for (var i = 0, limit = 100; i < limit; i++) {
    // This block will be executed 100 times
    console.log('Currently at ' + i);
    // Note: the last log will be "Currently at 99"
}

The while loop

A while loop is similar to an if statement, except that its body will keep executing until the condition evaluates to false.

while ([conditional]) [loopBody]

Here’s a typical while loop:

A typical while loop

var i = 0;
while (i < 100) {

    // This block will be executed 100 times
    console.log('Currently at ' + i);

    i++; // increment i

}

You’ll notice that we’re having to increment the counter within the loop’s body. It is possible to combine the conditional and incrementer, like so:

A while loop with a combined conditional and incrementer

var i = -1;
while (++i < 100) {
    // This block will be executed 100 times
    console.log('Currently at ' + i);
}

Notice that we’re starting at -1 and using the prefix incrementer (++i).

The do-while loop

This is almost exactly the same as the while loop, except for the fact that the loop’s body is executed at least once before the condition is tested.

do [loopBody] while ([conditional])

Here’s a do-while loop:

A do-while loop

do {

    // Even though the condition evaluates to false
    // this loop's body will still execute once.

    alert('Hi there!');

} while (false);

These types of loops are quite rare since only few situations require a loop that blindly executes at least once. Regardless, it’s good to be aware of it.

Breaking and continuing

Usually, a loop’s termination will result from the conditional statement not evaluating to true, but it is possible to stop a loop in its tracks from within the loop’s body with the break statement.

Stopping a loop

for (var i = 0; i < 10; i++) {
    if (something) {
        break;
    }
}

You may also want to continue the loop without executing more of the loop’s body. This is done using the continue statement.

Skipping to the next iteration of a loop

for (var i = 0; i < 10; i++) {

    if (something) {
        continue;
    }

    // The following statement will only be executed
    // if the conditional 'something' has not been met
    console.log('I have been reached');

}

Reserved Words

JavaScript has a number of “reserved words,” or words that have special meaning in the language. You should avoid using these words in your code except when using them with their intended meaning.

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

Arrays

Arrays are zero-indexed 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.

A simple array

var myArray = [ 'hello', 'world' ];

Accessing array items by index

var myArray = [ 'hello', 'world', 'foo', 'bar' ];
console.log(myArray[3]);   // logs 'bar'

Testing the size of an array

var myArray = [ 'hello', 'world' ];
console.log(myArray.length);   // logs 2

Changing the value of an array item

var myArray = [ 'hello', 'world' ];
myArray[1] = 'changed';

While it’s possible to change the value of an array item as shown in “Changing the value of an array item”, it’s generally not advised.

Adding elements to an array

var myArray = [ 'hello', 'world' ];
myArray.push('new');

Working with arrays

var myArray = [ 'h', 'e', 'l', 'l', 'o' ];
var myString = myArray.join('');   // 'hello'
var mySplit = myString.split('');  // [ 'h', 'e', 'l', 'l', 'o' ]

Objects

Objects contain one or more key-value pairs. The key portion can be any string. The value portion can be any type of value: a number, a string, an array, a function, or even another object.

[Definition: When one of these values is a function, it’s called a method of the object.] Otherwise, they are called properties.

As it turns out, nearly everything in JavaScript is an object — arrays, functions, numbers, even strings — and they all have properties and methods.

Creating an “object literal”

var myObject = {
    sayHello : function() {
        console.log('hello');
    },
    myName : 'Rebecca'
};
myObject.sayHello();            // logs 'hello'
console.log(myObject.myName);   // logs 'Rebecca'

Note

When creating object literals, you should note that the key portion of each key-value pair can be written as any valid JavaScript identifier, a string (wrapped in quotes) or a number:

var myObject = {
    validIdentifier: 123,
    'some string': 456,
    99999: 789
};

Object literals can be extremely useful for code organization; for more information, read Using Objects to Organize Your Code by Rebecca Murphey.

Functions

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:

Function Declaration

function foo() { /* do something */ }

Named Function Expression

var foo = function() { /* do something */ }

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.

Using Functions

A simple function

var greet = function(person, greeting) {
    var text = greeting + ', ' + person;
    console.log(text);
};
greet('Rebecca', 'Hello');

A function that returns a value

var greet = function(person, greeting) {
    var text = greeting + ', ' + person;
    return text;
};

console.log(greet('Rebecca','hello'));

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();

Self-Executing Anonymous Functions

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.

A self-executing anonymous function

(function(){
    var foo = 'Hello world';
})();


console.log(foo);   // undefined!

Functions as Arguments

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.

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'

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'

Testing Type

JavaScript offers a way to test the “type” of a variable. However, the result can be confusing — for example, the type of an Array is “object”.

It’s common practice to use the typeof operator when trying to determining the type of a specific value.

Testing the type of various variables

var myFunction = function() {
    console.log('hello');
};

var myObject = {
    foo : 'bar'
};

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

var myString = 'hello';

var myNumber = 3;

typeof myFunction;   // returns 'function'
typeof myObject;     // returns 'object'
typeof myArray;      // returns 'object' -- careful!
typeof myString;     // returns 'string';
typeof myNumber;     // returns 'number'

typeof null;         // returns 'object' -- careful!


if (myArray.push && myArray.slice && myArray.join) {
    // probably an array
    // (this is called "duck typing")
}

if (Object.prototype.toString.call(myArray) === '[object Array]') {
    // Definitely an array!
    // This is widely considered as the most robust way
    // to determine if a specific value is an Array.
}

jQuery offers utility methods to help you determine the type of an arbitrary value. These will be covered later.

The `this` keyword

In JavaScript, as in most object-oriented programming languages, this is a special keyword that is used within methods to refer to the object on which a method is being invoked. The value of this is determined using a simple series of steps:

If the function is invoked using Function.call or Function.apply, this will be set to the first argument passed to call/apply. If the first argument passed to call/apply is null or undefined, this will refer to the global object (which is the window object in Web browsers).
If the function being invoked was created using Function.bind, this will be the first argument that was passed to bind at the time the function was created.
If the function is being invoked as a method of an object, this will refer to that object.
Otherwise, the function is being invoked as a standalone function not attached to any object, and this will refer to the global object.

A function invoked using Function.call

var myObject = {
    sayHello : function() {
        console.log('Hi! My name is ' + this.myName);
    },

    myName : 'Rebecca'
};

var secondObject = {
    myName : 'Colin'
};

myObject.sayHello();                  // logs 'Hi! My name is Rebecca'
myObject.sayHello.call(secondObject); // logs 'Hi! My name is Colin'

A function created using Function.bind

var myName = 'the global object',

    sayHello = function () {
        console.log('Hi! My name is ' + this.myName);
    },

    myObject = {
        myName : 'Rebecca'
    };

var myObjectHello = sayHello.bind(myObject);

sayHello();       // logs 'Hi! My name is the global object'
myObjectHello();  // logs 'Hi! My name is Rebecca'

A function being attached to an object at runtime

var myName = 'the global object',

    sayHello = function() {
        console.log('Hi! My name is ' + this.myName);
    },

    myObject = {
        myName : 'Rebecca'
    },

    secondObject = {
        myName : 'Colin'
    };

myObject.sayHello = sayHello;
secondObject.sayHello = sayHello;

sayHello();               // logs 'Hi! My name is the global object'
myObject.sayHello();      // logs 'Hi! My name is Rebecca'
secondObject.sayHello();  // logs 'Hi! My name is Colin'

Note

When invoking a function deep within a long namespace, it is often tempting to reduce the amount of code you need to type by storing a reference to the actual function as a single, shorter variable. It is important not to do this with instance methods as this will cause the value of this within the function to change, leading to incorrect code operation. For instance:

var myNamespace = {
    myObject : {
        sayHello : function() {
            console.log('Hi! My name is ' + this.myName);
        },

        myName : 'Rebecca'
    }
};

var hello = myNamespace.myObject.sayHello;

hello();  // logs 'Hi! My name is undefined'

You can, however, safely reduce everything up to the object on which the method is invoked:

var myNamespace = {
    myObject : {
        sayHello : function() {
            console.log('Hi! My name is ' + this.myName);
        },

        myName : 'Rebecca'
    }
};

var obj = myNamespace.myObject;

obj.sayHello();  // logs 'Hi! My name is Rebecca'

Scope

“Scope” refers to the variables that are available to a piece of code at a given time. A lack of understanding of scope can lead to frustrating debugging experiences.

When a variable is declared inside of a function using the var keyword, it is only available to code inside of that function — code outside of that function cannot access the variable. On the other hand, functions defined inside that function will have access to to the declared variable.

Furthermore, variables that are declared inside a function without the var keyword are not local to the function — JavaScript will traverse the scope chain all the way up to the window scope to find where the variable was previously defined. If the variable wasn’t previously defined, it will be defined in the global scope, which can have extremely unexpected consequences;

Functions have access to variables defined in the same scope

var foo = 'hello';

var sayHello = function() {
    console.log(foo);
};

sayHello();         // logs 'hello'
console.log(foo);   // also logs 'hello'

Code outside the scope in which a variable was defined does not have access to the variable

var sayHello = function() {
    var foo = 'hello';
    console.log(foo);
};

sayHello();         // logs 'hello'
console.log(foo);   // doesn't log anything

Variables with the same name can exist in different scopes with different values

var foo = 'world';

var sayHello = function() {
    var foo = 'hello';
    console.log(foo);
};

sayHello();         // logs 'hello'
console.log(foo);   // logs 'world'

Functions can “see” changes in variable values after the function is defined

var myFunction = function() {
    var foo = 'hello';

    var myFn = function() {
        console.log(foo);
    };

    foo = 'world';

    return myFn;
};

var f = myFunction();
f();  // logs 'world' -- uh oh

Scope insanity

// a self-executing anonymous function
(function() {
    var baz = 1;
    var bim = function() { alert(baz); };
    bar = function() { alert(baz); };
})();

console.log(baz);  // baz is not defined outside of the function

bar();  // bar is defined outside of the anonymous function
        // because it wasn't declared with var; furthermore,
        // because it was defined in the same scope as baz,
        // it has access to baz even though other code
        // outside of the function does not

bim();  // bim is not defined outside of the anonymous function,
        // so this will result in an error

Closures

Closures are an extension of the concept of scope — functions have access to variables that were available in the scope where the function was created. If that’s confusing, don’t worry: closures are generally best understood by example.

In “Functions can”see" changes in variable values after the function is defined”, we saw how functions have access to changing variable values. The same sort of behavior exists with functions defined within loops — the function “sees” the change in the variable’s value even after the function is defined, resulting in all clicks alerting 5.

How to lock in the value of i?

/* this won't behave as we want it to; */
/* every click will alert 5 */
for (var i=0; i<5; i++) {
    $('<p>click me</p>').appendTo('body').click(function() {
        alert(i);
    });
}

Locking in the value of i with a closure

/* fix: 'close' the value of i inside
        createFunction, so it won't change */
var createFunction = function(i) {
    return function() { alert(i); };
};

for (var i=0; i<5; i++) {
    $('<p>click me</p>').appendTo('body').click(createFunction(i));
}

Closures can also be used to resolve issues with the this keyword, which is unique to each scope:

Using a closure to access inner and outer object instances simultaneously

var outerObj = {
    myName : 'outer',
    outerFunction : function () {

        // provide a reference to outerObj
        // through innerFunction's closure
        var self = this;

        var innerObj = {
            myName : 'inner',
            innerFunction : function () {
                // logs 'outer inner'
                console.log(self.myName, this.myName);
            }
        };

        innerObj.innerFunction();

        console.log(this.myName); // logs 'outer'
    }
};

outerObj.outerFunction();

This mechanism can be particularly useful when dealing with callbacks, though in those cases, it is often better to use Function.bind, which will avoid any overhead associated with scope traversal.

JavaScript Basics

Based on “jQuery Fundamentals” by Rebecca Murphey

http://github.com/rmurphey/jqfundamentals

Creative Commons Attribution-Share Alike 3.0 US

Overview

Syntax Basics

Operators

Basic Operators

Operations on Numbers & Strings

Logical Operators

Note

Comparison Operators

Conditional Code

Note

Truthy and Falsy Things

Conditional Variable Assignment with The Ternary Operator

Switch Statements

Loops

The for loop

The while loop

The do-while loop

Breaking and continuing

Reserved Words

Arrays

Objects

Note

Functions

Using Functions

Self-Executing Anonymous Functions

Functions as Arguments

Testing Type

The `this` keyword

Note

Scope

Closures