Javascript

// 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 global-scoped variable and assigns 3.  This is generally considered // bad practice, and will not work if strict mode is on. t = 3;  // 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";