Like most languages, variables in Javascript are defined with the = operator.

There are 3 different keywords that can be used to define a variable.

  • var - Depricated, should not be used for ES6+.
  • let - Variable can be re-assigned. Value can be mutated.
  • const - Variable cannot be re-assigned. Value can be mutated.

Note

let and const have been available since ES6/2015.

Define a Variable

javascript

// var syntax (considered depricated)
var name = "stuff";

// let syntax
// initialize variable without value assignment
// only available with "let" keyword
let name;

// initialize a variable and assign a value
let name = "stuff";

// re-assign a variable
let name = "things";

// const syntax
// initialize a constant and assign a value
// a value must be assigned when initialized
const name = "stuff";

// re-assigning a constant will result in a runtime error
const name = "things"; // => Uncaught SyntaxError: redeclaration of var name
    

Considerations

  • camelCase is the convention used for multi-word variables.
  • Variables can contain letters, numbers, underscores _ and a dollar sign $.
  • Variables cannot start with a number.
  • let should be used when a variable may be overwritten such as in a for loop.
  • const should be used when a variable should not be overwritten.

Tags

javascript typescript