Variables

A "variable" is a place where you can store information, such as a string, or a number. A variable has a name (that you choose) and a value. New variables in JavaScript are declared using one of three keywords: let, const, or var.

Think of variables names like labels on boxes, while the value of the variable are the contents of the box - you could change the contents of a box and leave the label intact, the contents of the boxes can have different types, the boxes should have good labels (a box of books being labeled pens would be very confusing),

Photo from Khan Academy

Variables are "declared" using the var, let or const keyword. In the following example three variables are declared with the names x, foo and bar.

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var x;
let foo;
const bar;

Note that the chosen names in this example are meaningless (perhaps with the exception of x, for instance as part of a mathematical program). You should make an effort to always choose names that best describe what you intend this variable to hold.

Prior to JavaScript ES6 the var keyword was the only way to declare a variable. ES6 introduced two new keywords, let and const for declaring variables. They improve on how the older var declaration works (this involves the concept of "scope" that you will learn about in the third lecture). In HackYourFuture we encourage you to use the more modern let and const keywords over var, but you will often come across var in existing books, software libraries and examples on the Internet, so you should understand var too.

• read about let
• read about const
• let vs const

Here, we say: "declare variable x and initialize it with the integer (number) 6".

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let foo; // declare variable `foo`

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let foo = 6; // declare and assign a variable at the same time

You can also assign a value to an existing variable:

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foo = 4; // change variable `foo`