Variables and Constants

Variables in JavaScript can be declared in two ways:

• var declares a function-scoped or globally-scoped variable, optionally initializing it to a value
• let declares a block-scoped local variable, optionally initializing it to a value

Constants are declared via the keyword const.

var keyword

The scope of a variable declared with var is its current execution context and closures thereof, which is either the enclosing function and functions declared within it, or, for variables declared outside any function, global. Duplicate variable declarations using var will not trigger an error, even in strict mode, and the variable will not lose its value, unless another assignment is performed.

The var keyword is not bad per se, but there is no need to use var in new projects.

let keyword

let allows you to declare variables that are limited to the scope of a block statement, or expression on which it is used, unlike the var keyword, which declares a variable globally, or locally to an entire function regardless of block scope. The other difference between var and let is that the latter is initialized to a value only when a parser evaluates it (see below).

const keyword

const declares a constant whose value you want to forbid overwriting. Constants are block-scoped, much like variables declared using the let keyword. If a constant is an object or array its properties or items can still be updated or removed.

Variables without a keyword

Variables can also be declared without a keyword:

age = 21;

When this is done, the variable is implicitly declared globally. This is bad practice and should be avoided.