“Undefined” In Javascript

“undefined” through example with a simple function

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

Function

function undefinedOrNot(value) {
  if (value === undefined) {
    return 'error';
  } else {
    return value;
  }
}

1. Input undefined

undefinedOrNot(undefined)
// "error"

2. Input no parameter

undefinedOrNot()
// "error"

3. Input a string parameter

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

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