What is the difference between null, undefined, and undeclared in JavaScript?

JavaScript

undefined

  • A variable has been declared but no value has been assigned to it
  • It's the default value of function parameters and variables that are declared but not initialized
let x;
console.log(x); // undefined
function test(param) {
  console.log(param); // undefined if no argument is passed
}
let c = undefined;
console.log(c); // undefined
let d = {};
console.log(d.fake); // undefined

null

  • Represents an intentional absence of any object value
  • It's a primitive value that represents "nothing", "empty" or "value unknown"
  • Must be explicitly assigned
let user = null;
console.log(user); // null
console.log(typeof null); // "object" (this is a known JavaScript quirk)

undeclared

  • Variables that don't exist and haven't been declared with var, let, or const
  • Trying to access them throws a ReferenceError
console.log(nonExistentVariable);
// Throws ReferenceError: nonExistentVariable is not defined

Practical difference between undefined and null

let logHi = (str = 'hi') => {
  console.log(str);
};

The code above creates a function named logHi. This function requires one parameter and sets the default of that parameter to hi if it isn’t supplied. Here’s what that looks like:

logHi(); // hi

We can also supply a parameter to overwrite this default:

logHi('bye'); // bye

With default parameter, undefiend will use the default while null does not.

logHi(undefined); // hi
logHi(null); // null

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