JS Variable Keywords - kimschles/schlesinger-knowledge GitHub Wiki

const, let, var

Resources

• Understanding Variables, Scope, and Hoisting in JavaScript by Tania Rascia
• fun fun function video

Variables

• Variables provide developers a way to store a value
• Once a variable is declared, the name of the variable is called an identifier
• For example, const name = 'kim'; `name is the identifier
• In JavaScript, variable identifiers can contain the letters (a-z), numbers (0-9), underscores (_) and the dollar sign ($)

Explanations and Use Cases

const

• Used when the value of the variable will stay the same
• Throws an error if there is an attempt at updating the value
• Block scope: can see/has access to only what is inside the curly braces. Can be a function, loop or conditional
• Need to be declared and initialized at the same time because you cannot reassign a value
• ALL_CAPS or lowercase? Application level constants yes, not when you use const for declaring immutable variables in function scope.
• Use const as often as possible to minizimize mutable state

Example:

const API_URL = asdlkfjasldkfjasdf.com/v1/plants;

function filterData(){
    const result = {};
    // code here
    return resut;
}

let

• Used when the value of the variable will be reassigned
• Most commonly used in for loops
• Block scope: can see/access what is inside the curly braces. can be a function, loop or conditional.

var

• In ES5 and prior versions of ECMAScript, var was the only variable declaration keyword
• Avoid var
• When is var used in ES6?
• function scope: can see/has access to anything within the curly braces of a function