JavaScript language overview JavaScript MDN

Companies can use the open standard language to develop their implementation of JavaScript. The ECMAScript standard is documented in the ECMA-262 specification. Developers can leverage these functions instead of writing code from scratch, saving valuable time.


As a scripting language, it plays an important role in web browsers, allowing real-time modification of web page content and enhancing user experience. A runtime, or a host, is something that feeds data to the JavaScript engine (the interpreter), provides extra global properties, and provides hooks for the engine to interact with the outside world. Module resolution, reading data, printing messages, sending network requests, etc. are all runtime-level operations. Since its inception, JavaScript has been adopted in various environments, such as browsers (which provide APIs like DOM), Node.js (which provides APIs like file system access), etc.

Comparing operands of the same type

JavaScript is a cross-platform, object-oriented scripting language used to make webpages interactive (e.g., having complex animations, clickable buttons, popup menus, etc.). Inside a host environment (for example, a web browser), JavaScript can be connected to the objects of its environment to provide programmatic control over them. Java is a class-based programming language designed for fast execution and type safety.


python for javascript developers classes are the most appropriate way to do object-oriented programming. Learn more about the behavior of JavaScript’s operators instanceof, typeof, new, this, the operator precedence, and more. Explanation of the widely misunderstood and underestimated prototype-based inheritance.

JavaScript Numbers

You’ll learn the syntax and usage of expressions, which are combinations of values, variables, operators, and functions that produce a result. Unlike objects in statically typed languages, objects in do not have fixed shapes — properties can be added, deleted, re-ordered, mutated, or dynamically queried at any time. Object keys are always strings or symbols — even array indices, which are canonically integers, are actually strings under the hood. Like in 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. In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE.

to provide some default value to another variable foo, you may encounter unexpected behaviors if you consider some falsy values as usable (e.g., ” or 0).

  • 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.
  • They handle repetitive tasks, allowing developers to focus on core functionality.
  • A common JavaScript-related security problem is cross-site scripting (XSS), a violation of the same-origin policy.
  • Notably, logical operators don’t work with boolean values only — they work by the “truthiness” of the value.
  • The core language specification focuses on pure computational logic. It doesn’t deal with any input/output — in fact, without extra runtime-level APIs (most notably console.log()), a JavaScript program’s behavior is entirely unobservable. However, the JavaScript language doesn’t offer standard library modules — all core functionalities are powered by global variables like Math and Intl instead.


    This example shows that, in JavaScript, function closures capture their non-local variables by reference. A nested function is a function defined within another function. In 2005, Mozilla joined ECMA International, and work started on the ECMAScript for XML (E4X) standard. This led to Mozilla working jointly with Macromedia (later acquired by Adobe Systems), who were implementing E4X in their ActionScript 3 language, which was based on an ECMAScript 4 draft.

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