JS : What is JavaScript ?

JavaScript (JS) is an interpreted computer programming language. It was originally implemented as part of web browsers so that client-side scripts may interact with the user, control the browser, communicate asynchronously and alter the document content that is displayed.
JavaScript is prototype-based scripting language that is dynamic, weakly typed and has first-class functions.
It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.

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-level scoping is not supported (instead, JavaScript has function-level scoping). JavaScript 1.7, however, supports block-level scoping with the let keyword. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, in which the semicolons that terminate statements can be omitted.

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.[25]
object based
JavaScript is almost entirely object-based. JavaScript objects are associative arrays, augmented with prototypes (see below). Object property names are string keys: obj.x = 10 and obj[‘x’] = 10 are equivalent, the dot notation being syntactic sugar. 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.

first-class functions
Functions are first-class; they are objects themselves. As such, they have properties and methods, such as .call() and .bind(); and they can be assigned to variables, passed as arguments, returned by other functions, and manipulated like any other object. Any reference to a function allows it to be invoked using the () operator.
nested functions and closures
“Inner” or “nested” functions are functions defined within another function. They are created each time the outer function is invoked. In addition to that, each created function forms a lexical closure: the lexical scope of the outer function, including any constants, local variables and argument values, become part of the internal state of each inner function object, even after execution of the outer function concludes.

JavaScript uses prototypes instead of 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 creates a new object and calls that function with its local this keyword bound to that object for that invocation. The constructor’s prototype property determines the object used for the new object’s internal prototype. JavaScript’s built-in constructors, such as Array, also have prototypes that can 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; a function can be called as a method. 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.

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”. In fact, it relies on the environment to provide the ability to includeimport 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.
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.

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