JavaScript is a scripting language widely used for client-side web development. It was the originating dialect of the ECMAScript standard. It is a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to look like Java, but be easier for non-programmers to work with.^[1][2]

Although best known for its use in websites (as client-side JavaScript), JavaScript is also used to enable scripting access to objects embedded in other applications .

JavaScript, despite the name, is essentially unrelated to the Java programming language, although both have the common C syntax, and JavaScript copies many Java names and naming conventions. The language’s name is the result of a co-marketing deal between Netscape and Sun, in exchange for Netscape bundling Sun’s Java runtime with their then-dominant browser. The key design principles within JavaScript are inherited from the Self and Scheme programming languages.^[3]

“JavaScript” is a trademark of Sun Microsystems. It was used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.^[4]

History and naming

JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript.^[5] The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web browser. JavaScript was first introduced and deployed in the Netscape browser version 2.0B3 in December 1995. The naming has caused confusion, giving the impression that the language is a spin-off of Java, and it has been characterized by many as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web-programming language.

Due to the widespread success of JavaScript as a client-side scripting language for web pages, Microsoft developed a compatible dialect of the language, naming it JScript to avoid trademark issues. JScript added new date methods to fix the non-Y2K-friendly methods in JavaScript, which were based on java.util.Date.^[2] JScript was included in Internet Explorer 3.0, released in August 1996. The dialects are perceived to be so similar that the terms “JavaScript” and “JScript” are often used interchangeably. Microsoft, however, notes dozens of ways in which JScript is not ECMA compliant.

Netscape submitted JavaScript to Ecma International for standardization resulting in the standardized version named ECMAScript.

The flexibility of JavaScript has made it one of the most popular programming languages on the web and also one of the easier languages to learn. Initially, however, many professional programmers denigrated the language because its target audience was web authors and other such “amateurs”, among other reasons.^[9] The advent of AJAX returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive frameworks and libraries, improved JavaScript programming practices, and increased usage of JavaScript outside of the web.

JavaScript supports all the structured programming syntax in C (e.g., if statements, while loops, switch statements, etc.). One partial exception is scoping: C-style block-level scoping is not supported. JavaScript 1.7, however, supports block-level scoping with the let keyword. Like C, JavaScript makes a distinction between expressions and statements.

Dynamic

dynamic typing

As in most scripting languages, types are associated with values, not variables. For example, a variable x could be bound to a number, then later rebound to a string. JavaScript supports various ways to test the type of an object, including duck typing.^[10]

objects as associative arrays

JavaScript is almost entirely object-based. Objects are associative arrays, augmented with prototypes (see below). Object property names are associative array keys: obj.x = 10 and obj["x"] = 10 are equivalent, the dot notation being merely syntactic sugar. Properties and their values can be added, changed, or deleted at run-time. The properties of an object can also be enumerated via a for...in loop.

run-time evaluation

JavaScript includes an eval function that can execute statements provided as strings at run-time.

Functional

first-class functions

Functions are first-class; they are objects themselves. As such, they have properties and can be passed around and interacted with like any other object.

inner functions and closures

Inner functions (functions defined within other functions) are created each time the outer function is invoked, and variables of the outer functions for that invocation continue to exist as long as the inner functions still exist, even after that invocation is finished (e.g. if the inner function was returned, it still has access to the outer function’s variables) — this is the mechanism behind closures within JavaScript.

Prototype-based

prototypes

JavaScript uses prototypes instead of classes for defining object properties, including methods, and inheritance. It is possible to simulate many class-based features with prototypes in JavaScript.

functions as object constructors

Functions double as object constructors along with their typical role. Prefixing a function call with new creates a new object and calls that function with its local this keyword bound to that object for that invocation. The function’s prototype property determines the new object’s prototype.

functions as methods

Unlike many object-oriented languages, there is no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; a function can be called as a method. When a function is invoked as a method of an object, the function’s local this keyword is bound to that object for that invocation.

Miscellaneous

run-time environment

JavaScript typically relies on a run-time environment (e.g. in a web browser) to provide objects and methods by which scripts can interact with “the outside world”. (This is not a language feature per se, but it is common in most JavaScript implementations.)

variadic functions

An indefinite number of parameters can be passed to a function. The function can both access them through formal parameters and the local arguments object.

array and object literals

Like many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form the basis of the JSON data format.

regular expressions

JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.

function LCMCalculator(x, y) { // constructor function
function checkInt(x) { // inner function
if (x % 1 != 0)
throw new TypeError(x + ” is not an integer”); // exception throwing
return x;
}
this.a = checkInt(x);
this.b = checkInt(y);
this.ab = this.a * this.b;
}
LCMCalculator.prototype = { // object literal for constructor’s prototype
gcd : function() {
// Euclidean algorithm:
var a = Math.abs(this.a), b = Math.abs(this.b);
if (a < b) {
var t = b; b = a; a = t; // swap variables
}
while (b != 0) {
t = b; // |t| already declared above (though we could redeclare if we wish)
b = a % b;
a = t;
}
// Only need to calculate gcd once, so redefine this method.
// ‘gcd’ == “gcd”, this['gcd'] == this.gcd
this['gcd'] = function() { return a; };
return a;
},
lcm : function() {
// Variable names don’t collide with object properties, e.g. |lcm| is not |this.lcm|.
var lcm = this.ab / this.gcd();
// Only need to calculate lcm once, so redefine this method.
this.lcm = function() { return lcm; };
return lcm;
},
toString : function() {
return “LCMCalculator: a = ” + this.a + “, b = ” + this.b;
}
};
[[25,55],[21,56],[22,58],[28,56]].map(function(pair) { // array literal + mapping function
return new LCMCalculator(pair[0], pair[1]);
}).sort(function(a, b) { // comparative function
return a.lcm() – b.lcm();
}).forEach(function(obj) {
/* Note: println() is not an actual JS function, but let’s assume
* it’s functionally equivalent to Java’s System.out.println().
*/
println(obj + “, gcd = ” + obj.gcd() + “, lcm = ” + obj.lcm());
});
// Note: Array’s map() and forEach() is predefined in JavaScript 1.6.
The output is:
LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56
LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168
LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275
LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638