javascript check if variable has a value number code example

Example 1: How can I test whether a variable has a value in JavaScript?

if (typeof variable == "undefined") {...} --> return true if variable undefined

//An empty string evaluates to FALSE in JavaScript so you can just do:
if (variable) {...}

Example 2: check if a variable is a number in javascript

// as test cases we have two variables of which 1 is a number and the second one is a string
let x = 7;
let y = "hello"
let result1 = !Number.isNaN(x) // true
let result2 = !Number.isNaN(y) // flase