Main differences from other languages

Prototypal inheritance

While the ES2015 standard brought the class keyword to Javascript, these classes are not an implementation of the classical inheritance, but rather just syntactic sugar for Javascript’s actual inheritance model — prototypal inheritance.

Simply put, prototypal inheritance differs from classical inheritance in the sense that while classical inheritance uses “blueprints”, i.e. classes, from which objects are instantiated, in prototypal inheritance, such blueprints do not exists. In Javascript, objects inherit directly from other objects, which thus become prototypes of the former.

This also means that Javascript uses prototype chains instead of extended classes, and that it does not support multiple inheritance at the same hierarchical level of the parent. A very common pattern to achieve most benefits of multiple inheritance while avoiding some pitfalls thereof, however, is concatenative inheritance.

Functional

Javascript is a multi-paradigm language. It can be used to write both Object-Oriented or Functional code. This is possible because functions are 1st class citizens in Javascript, which means they can be used and passed around as any other value could be. This is aided by the concepts of lexical scope and lexical closures, which enable internal function state preservation even after execution of the outer scope(s) finishes.

Asynchronous

Where most other mainstream languages rely on multi-threading, Javascript instead relies on the internal Event-Loop and uses web workers and child processes to make use of multiprocessor systems. This allows Javascript to avoid many common problems arising from bad thread handling without major performance sacrifice. The asynchronicity also prevents IO-blocking, which is especially crucial in web browsers, where a blocked UI thread could completely ruin the UX.

Variadic functions

Where most languages demand one specify the exact number and often type of function parameters, Javascript allows for passing a smaller or greater number of parameters to functions than explicitly specified. This is highly useful when needing to “pre-load” a function with e.g. just the first two parameters, which binds back to Functional Programming concepts.