js define if undefined code example

Example 1: checking if var is not defined js

if(typeof x !== 'undefined'){
  alert("Variable x is defined.");
}

Example 2: assign this value or if it is undefined this other value javascript

var x; //variable to change
var y; //higher priority variable
var z = 12; //lower priority variable
x = !(y === undefined) ? y : z;
console.log(x); //12