null vs undefined

Learn the difference between null and undefined.

Difference between null and undefined

JavaScript and TypeScript have two distinct "no value" states. undefined means a variable has been declared but its value has not been assigned yet. null is an empty value the programmer sets on purpose.

let a: string | null = null;
let b: string | undefined = undefined;
let c: string | undefined;
console.log(c);  // undefined

So undefined means "not yet", null means "intentionally empty".

strictNullChecks

By default, TypeScript treats null and undefined as assignable to any other type. With the strictNullChecks option on, they become separate types; you cannot assign null to a string variable, but writing string | null explicitly is valid.

let s: string = "hi";
let maybe: string | null = null;

This option lets you catch runtime errors at compile time.

Exercise Write greet with a null check

Write a function greet(name: string | null): string. If name is null, return "yok"; otherwise return name.toUpperCase(). Print the result of both calls.

Show example solution

function greet(name: string | null): string {
  if (name === null) {
    return "yok";
  }
  return name.toUpperCase();
}

console.log(greet("ada"));
console.log(greet(null));

Test yourself

What is the value of a variable that has been declared but not assigned?
- empty string
- undefined
- null
- 0
Show answer

undefined — In TypeScript, a declared but unassigned variable is automatically undefined. null is a value the programmer assigns on purpose.

Add null to the union

let userName: string ____ = null;

Show answer

let userName: string | null = null;

null and undefined parts

null

Intentional empty value
undefined

Unassigned variable
string | null

Can be string or null
strictNullChecks

Makes null and undefined separate types

Next lesson: Optional chaining

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