strictNullChecks

Learn the option that enforces null safety.

What is strictNullChecks?

With strictNullChecks on, null and undefined become separate from other types. You cannot assign null to a string variable; you must write string | null for it to accept null. This prevents runtime null-read errors.

let t: string | null = null;
if (t !== null) {
  console.log(t.length);  // safe access
}

When strictNullChecks is off

With strictNullChecks off, null and undefined are assignable to any type. This flexibility is dangerous; if you access a property without checking for null, the app may crash.

let s: string = null;  // valid when off
console.log(s.length);  // runtime error

With it on, this line fails at compile time and you see the error early.

Exercise Null-safe first element

Write a function firstOrNull(arr: string[]): string | null. If the array is empty, return null; otherwise return the first element. Print the result of both calls.

Show example solution

function firstOrNull(arr: string[]): string | null {
  if (arr.length === 0) {
    return null;
  }
  return arr[0];
}

console.log(firstOrNull(["a", "b"]));
console.log(firstOrNull([]));

Test yourself

Which assignment is valid when strictNullChecks is on?
- let s: string = null;
- let s: string; s = null;
- let s: string = undefined;
- let s: string | null = null;
Show answer

let s: string | null = null; — With strictNullChecks on, you cannot assign null or undefined to a string variable. Writing string | null explicitly makes the null assignment valid.

Nullable return type

function getName(): string ____ { return null; }

Show answer

function getName(): string | null { return null; }

strictNullChecks parts

strictNullChecks

Makes null and undefined separate types
string | null

A string that can be null
string

Cannot be null under strict mode
t !== null

Type narrowing

Next lesson: strictFunctionTypes

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