JavaScript (JS) - is a dynamic computer programming language. It is most commonly used as part of web browsers, whose implementations allow client-side scripts to interact with the user, control the browser, communicate asynchronously, and alter the document content that is displayed. It is also being used in server-side programming, game development and the creation of desktop and mobile applications.

JavaScript is a prototype-based scripting language with dynamic typing and has first-class functions. Its syntax was influenced by C. JavaScript copies many names and naming conventions from Java, but the two languages are otherwise unrelated and have very different semantics. The key design principles within JavaScript are taken from the Self and Scheme programming languages. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.

Features

Imperative and structured

JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements, etc.). One partial exception is scoping: C-style block scoping is not supported. Instead, JavaScript has function scoping (although, block scoping using the let keyword was added in JavaScript 1.7). Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, which allows the semicolons that would normally terminate statements to be omitted.

Dynamic

Dynamic typing

As in most scripting languages, types are associated with values, not with 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.

Object-based

JavaScript is almost entirely object-based. JavaScript objects are associative arrays, augmented with prototypes (see below). Object property names are string keys. They support two equivalent syntaxes: dot notation (obj.x = 10) and bracket notation (obj['x'] = 10). Properties and their values can be added, changed, or deleted at run-time. Most properties of an object (and those on its prototype inheritance chain) can be enumerated using a for...in loop. JavaScript has a small number of built-in objects such as Function and Date.

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 methods, such as .call() and .bind().[35] A nested function is a function defined within another function. It is created each time the outer function is invoked. In addition, each created function forms a lexical closure: the lexical scope of the outer function, including any constants, local variables and argument values, becomes part of the internal state of each inner function object, even after execution of the outer function concludes.[36]

Prototype-based

Prototypes

JavaScript uses prototypes where many other object oriented languages use classes for 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 will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the Object prototype). ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null). The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the object used as a constructor. JavaScript's built-in constructors, such as Array or Object, also have prototypes that can be modified. While it is possible to modify the Object prototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from the Object prototype and they may not expect the prototype to be modified.

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; when a function is called as a method of an object, the function's local this keyword is bound to that object for that invocation.

Implicit and Explicit Delegation

JavaScript is a Delegation Language.

Functions as Roles (Traits and Mixins)

JavaScript natively supports various function based implementations of Role patterns like Traits and Mixins. Such a function defines additional behavior by at least one method bound to the this keyword within its function body. A Role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain.

Type Composition and Inheritance

Whereas explicit function based delegation does cover composition in JavaScript, implicit delegation already happens every time the prototype chain is walked in order to e.g. find a method that might be related to but is not directly owned by an object. Once the method was found it gets called within this object's context. Thus inheritance in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.

Miscellaneous

Run-time environment

JavaScript typically relies on a run-time environment (e.g. a web browser) to provide objects and methods by which scripts can interact with the environment (e.g. a webpage DOM). It also relies on the run-time environment to provide the ability to include/import scripts (e.g. HTML <script> elements). 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 access them through formal parameters and also through the local arguments object. Variadic functions can also be created by using the apply method.

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.