Client Side Scripting Using JavaScript

JavaScript code is embedded in a web page using the <SCRIPT> tag. Its Language attribute names the scripting language being used — here, always JavaScript. A page containing JavaScript is still saved with an .html extension.

document.write() is a JavaScript function that inserts text into the body section of the page — document refers to the body section itself. Like C++, every JavaScript statement ends in a semicolon, and script code can appear more than once, anywhere between HTML tags.

JavaScript is case sensitive — keywords are always lowercase. Identifiers conventionally use camelCase (e.g. firstName, checkData): lowerCamelCase when the first word starts lowercase, UpperCamelCase when it doesn't. Every browser has a JavaScript engine, a virtual machine that interprets script code at runtime; a page with no script renders faster since the browser skips handing anything to the engine. Users can disable JavaScript entirely in their browser settings, in which case the browser simply ignores everything inside <SCRIPT>.

A function is a named group of instructions, defined with the keyword function. Unlike C++, a JavaScript function declares no return type. Defining a function does not run it — it must be called by name, followed by parentheses and a semicolon.

A function can be defined inside <HEAD> or <BODY>, but placing it in the head section is usual practice — the head loads before the body, so by the time the body calls the function, its definition is already in memory.

Where C++ has int, char, float, double and more, JavaScript reduces this to just three basic data types.

If true or false are written inside quotes, they become strings instead. JavaScript is case sensitive, so TRUE and FALSE are not valid booleans.

Where C++ uses int, float, char and so on, JavaScript declares every variable with the single keyword var. A declaration alone doesn't define a variable's type — that's decided only once a value is assigned. Several variables can share one var statement, separated by commas.

The built-in typeof() function reports a variable's type. A variable that's declared but never assigned a value has the special type undefined.

Almost every operator in JavaScript works exactly as it does in C++.

The same + operator that adds numbers also concatenates strings — its behaviour depends on the type of its operands. If either operand is a string, both are treated as strings.

Number() converts a string containing digits into a true number type — essential when adding values pulled from a text box, since text box content is always treated as a string.

Every control structure familiar from C++ carries over unchanged.

A multi-branching statement — the matching case runs based on the value of the expression.

for uses a loop variable to control the number of iterations; while repeats for as long as its test expression stays true. do–while exists too, but for and while cover almost every case a beginner needs.

JavaScript's built-in functions (also called methods) handle common tasks without any code of your own.

The difference between a function and a property: a function is called with parentheses and often arguments (x.toUpperCase()); a property is accessed directly by name (x.length).

Any form control needs a Name before JavaScript can reach it. A control is then addressed as document.formName.elementName.value.

The function is wired to a control using an event attribute, most commonly onClick on a button:

A <SELECT> control's selectedIndex property gives the position of the chosen <OPTION>, counting from 0. Combined with isNaN(), string length checks, and return to bail out early, these pieces build real client-side validation.

JavaScript never raises an error dialog of its own — if the engine can't execute a line, it silently skips the rest of the function. That's why explicit checks with alert() and return matter for guiding the user.

Scripts can live in three places, each with a trade-off.

An external file holds JavaScript only — no <SCRIPT> tag inside it — and is linked in with the Src attribute. When Src is present, any code written inside that same <SCRIPT> tag is ignored, so a linked file and inline code need two separate tags.