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Variables in standard JavaScript have no type attached, so any value (each value has a type) can be stored in any variable. Starting with ES6, the 6th version of the language, variables could be declared with var for function scoped variables, and let or const which are for block level variables. Before ES6, variables could only be declared with a var statement. Values assigned to variables declared with const cannot be changed, but their properties can. var should no longer be used since let and const are supported by modern browsers.cite-ref-5[5] A variable's identifier must start with a letter, underscore (_), or dollar sign ($), while subsequent characters can also be digits (0-9). JavaScript is case sensitive, so the uppercase characters "A" through "Z" are different from the lowercase characters "a" through "z".

Starting with JavaScript 1.5, ISO 8859-1 or Unicode letters (or \uXXXX Unicode escape sequences) can be used in identifiers.cite-ref-6[6] In certain JavaScript implementations, the at sign (@) can be used in an identifier, but this is contrary to the specifications and not supported in newer implementations.

Variables declared with var are lexically scoped at a function level, while ones with let or const have a block level scope. Since declarations are processed before any code is executed, a variable can be assigned to and used prior to being declared in the code.cite-ref-7[7] This is referred to as hoisting, and it is equivalent to variables being forward declared at the top of the function or block.cite-ref-8[8]

With var, let, and const statements, only the declaration is hoisted; assignments are not hoisted. Thus a var x = 1 statement in the middle of the function is equivalent to a var x declaration statement at the top of the function, and an x = 1 assignment statement at that point in the middle of the function. This means that values cannot be accessed before they are declared; forward reference is not possible. With var a variable's value is undefined until it is initialized. Variables declared with let or const cannot be accessed until they have been initialized, so referencing such variables before will cause an error.

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