Variables and types

You can declare variables with

• let
• var
• const

let x = 10;
var y = 20;
const z = 2145325;

If a interface has a field that is not required, you can add a ? right in front of it

interface Useless
{
	possibleNumber? : 10 | 20;
}

speaking of that |, it shows that possibleNumber can either attain the value of 10 or 20. Refer to Union and Intersection Type Operators

interface Judgement
{
	formats?: '12h' | '24h';
}