JavaScript - Scope and Events

JavaScript Scope

Scope is the set of variables you have access to.

In JavaScript, objects and functions are also variables.

In JavaScript, scope is the set of variables, objects, and functions you have access to.

JavaScript has function scope: The scope changes inside functions.

Local JavaScript Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables have local scope: They can only be accessed within the function.

Example

// code here can not use carName

function myFunction() {
    var carName = "Volvo";

    // code here can use carName

}

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

Global JavaScript Variables

A variable declared outside a function, becomes GLOBAL.

A global variable has global scope: All scripts and functions on a web page can access it.

Example

var carName = " Volvo";

// code here can use carName

function myFunction() {

    // code here can use carName

}

Automatically Global

If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

This code example will declare carName as a global variable, even if it is executed inside a function.

Example

// code here can use carName

function myFunction() {
    carName = "Volvo";

    // code here can use carName

}

The Lifetime of JavaScript Variables

• The lifetime of a JavaScript variable starts when it is declared.
• Local variables are deleted when the function is completed.
• Global variables are deleted when you close the page.

Function Arguments

Function arguments (parameters) work as local variables inside functions.

Global Variables in HTML

With JavaScript, the global scope is the complete JavaScript environment.

In HTML, the global scope is the window object: All global variables belong to the window object.

Example

// code here can use window.carName

function myFunction() {
    carName = "Volvo";
}

JavaScript Events

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HTML events are "things" that happen to HTML elements.

When JavaScript is used in HTML pages, JavaScript can "react" on these events.

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HTML Events

An HTML event can be something the browser does, or something a user does.

Here are some examples of HTML events:

• An HTML web page has finished loading
• An HTML input field was changed
• An HTML button was clicked

Often, when events happen, you may want to do something.

JavaScript lets you execute code when events are detected.

HTML allows event handler attributes, with JavaScript code, to be added to HTML elements.

With single quotes:

<some-HTML-element some-event='some JavaScript'>

With double quotes:

<some-HTML-element some-event="some JavaScript">

In the following example, an onclick attribute (with code), is added to a button element:

Example

<button onclick='getElementById("demo").innerHTML=Date()'>The time is?</button>

In the example above, the JavaScript code changes the content of the element with id="demo".

In the next example, the code changes the content of its own element (using this.innerHTML):

Example

<button onclick="this.innerHTML=Date()">The time is?</button>

	JavaScript code is often several lines long. It is more common to see event attributes calling functions:

Example

<button onclick="displayDate()">The time is?</button>

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Common HTML Events

Here is a list of some common HTML events:

Event	Description
onchange	An HTML element has been changed
onclick	The user clicks an HTML element
onmouseover	The user moves the mouse over an HTML element
onmouseout	The user moves the mouse away from an HTML element
onkeydown	The user pushes a keyboard key
onload	The browser has finished loading the page

The list is much longer.

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What can JavaScript Do?

Event handlers can be used to handle, and verify, user input, user actions, and browser actions:

• Things that should be done every time a page loads
• Things that should be done when the page is closed
• Action that should be performed when a user clicks a button
• Content that should be verified when a user inputs data
• And more ...

Many different methods can be used to let JavaScript work with events:

• HTML event attributes can execute JavaScript code directly
• HTML event attributes can call JavaScript functions
• You can assign your own event handler functions to HTML elements
• You can prevent events from being sent or being handled
• And more ...