“Undefined” In Javascript

Michael S
1 min readFeb 18, 2019


“undefined” through example with a simple function

Simple example when a function parameter is recognized as “undefined” in JS.


function undefinedOrNot(value) {
if (value === undefined) {
return 'error';
} else {
return value;
  1. Input undefined
// "error"

2. Input no parameter

// "error"

3. Input a string parameter

ensure("I am string")
// "I am string"

I am blogging what I’ve learned to find later here for myself. If you happened to read this shit and there is wrong information, it would be appreciated to add a comment below.