FEATURES

Imperative and structured

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.