JSFiddle - React, Tailwind, and code Playground

JavaScript

function Default(variable, new_value)
{
    if(new_value === undefined) { return (variable === undefined) ? null : variable; }
    return (variable === undefined) ? new_value : variable;
}

var a = 2, b = "hello", c = true, d;

var test = Default(a, 0),
    test2 = Default(b, "Hi"),
    test3 = Default(c, false),
    test4 = Default(d, "Hello world");

window.alert(test + "\n" + test2 + "\n" + test3 + "\n" + test4);