TypeError: "x" is (not) "y"

// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: foo is undefined

var foo = null;
foo.substring(1); // TypeError: foo is null

// Certain methods might require a specific type
var foo = {};
Symbol.keyFor(foo); // TypeError: foo is not a symbol

var foo = "bar";
Object.create(foo); // TypeError: "foo" is not an object or null

if (typeof foo !== "undefined") {
  // Now we know that foo is defined, we are good to go.
}