js if not undefined then only assign code example

Example 1: 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

Example 2: javascript set variable if not defined

var x = (typeof x === 'undefined') ? your_default_value : x;