.JS

Definition 1: JS Source code

A .JS file is a source code file for the JavaScript programming language. It is an extremely common language used by most websites, although its usage can be problematic. JavaScript is heavily inspired by Java, and beginning developers sometimes get the 2 confused.

Here is an example of a Hello World program in this language:

// Declares a function-scoped variable named `x`, and implicitly assigns the // special value `undefined` to it. Variables without value are automatically // set to undefined. var x; // Variables can be manually set to `undefined` like so var x2 = undefined; // Declares a block-scoped variable named `y`, and implicitly sets it to // `undefined`. The `let` keyword was introduced in ECMAScript 2015. let y; // Declares a block-scoped, un-reassignable variable named `z`, and sets it to // a string literal. The `const` keyword was also introduced in ECMAScript 2015, // and must be explicitly assigned to. // The keyword `const` means constant, hence the variable cannot be reassigned // as the value is `constant`. const z = "this value cannot be reassigned!"; // Declares a variable named `myNumber`, and assigns a number literal (the value // `2`) to it. let myNumber = 2; // Reassigns `myNumber`, setting it to a string literal (the value `"foo"`). // JavaScript is a dynamically-typed language, so this is legal. myNumber = "foo";

Definition 2: Pseudo-extension (null)

A pseudo-extension is a file format that is applied to a file, despite not having a purpose, other than to change the file name (such as E.JS) however this file extension is rarely used as a pseudo-extension

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