JS++ is a programming language for web development that extends JavaScript with a sound type system. It includes imperative, object-oriented, functional, and generic programming features. It is free and open-source software released under a BSD license.

JS++ first appeared on October 8, 2011. The modern implementation was announced at DeveloperWeek 2016 and released on May 31, 2016. The language is designed by Roger Poon and Anton Rapetov.

Since JS++ is a superset of JavaScript, declaring data types for variables is optional. However, when types are declared, the types are enforced at both compile time and runtime.

Type annotations in JS++ use the traditional C/C++ syntax:

Notably, this differs from TypeScript and ActionScript, which use a more verbose style:

The type system in JS++ is sound for ECMAScript and DOM API corner cases, including host objects, dynamic key-value pairs, Comet, JScript conditional compilation, dynamic return types, ActiveX, ECMAScript for XML, web browser garbage collector and cyclic reference counting bugs, conditional logic, and other edge and corner cases. This differs from other JavaScript supersets where types are optional and discarded at runtime via type erasure, such as in TypeScript.

JS++ can use JavaScript libraries using the one-line external statement as in the following example from the homepage of JS++:

While classes in JavaScript (ECMAScript 6) are syntactic sugar for prototypes under the hood, JS++ classes resemble the classes found in classical programming languages such as C++, Java, and C# in terms of memory layout, performance, and semantics. "Classes" are a static concept, and they cannot be altered at runtime (during program execution) as is the case for JavaScript, Smalltalk, Lisp, and TypeScript, which rely on prototypes. For example, private methods are private at both compile time and runtime, and external JavaScript objects cannot access private JS++ fields or methods—even if a reference to a JS++ object is obtained from JavaScript.