Functional

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.