In JavaScript, any value has a type assigned.
The typeof operator is a unary operator that returns a string representing the type of a variable.
Example usage:
typeof 1 //'number'
typeof '1' //'string'
typeof {name: 'Flavio'} //'object'
typeof [1, 2, 3] //'object'
typeof true //'boolean'
typeof undefined //'undefined'
typeof (() => {}) //'function'
typeof Symbol() //'symbol'
JavaScript has no "function" type, and it seems funny that typeof returns 'function' when we pass it a function.
It's one quirk of it, to make our job easier.
If you don't initialize the variable when you declare it, it will have the undefined value until you assign a value to it.
let a //typeof a === 'undefined'
typeof works also on object properties.
If you have a car object, with just one property:
const car = {
model: 'Fiesta'
}
This is how you check if the color property is defined on this object:
if (typeof car.color === 'undefined') {
// color is undefined
}