Example of if a variable is undefined

function test() {
    console.log(_.isUndefined(foo)); // true, no error due to hoisting
    var foo = 3;
    console.log(_.isUndefined(foo)); // false, foo is defined now.
}

Example of if a variable is null

function test() {
var foo = null;
console.log(_.isNull(foo)); // true,
var foo = 3;
console.log(_.isNull(foo)); // false,
}