strictNullChecks

null stops being in every type

Without it, `null` and `undefined` are members of every type, so `string` silently means "string, or nothing at all".

Since: TypeScript 2.0
strict: In strict
In your tsconfig: "strictNullChecks": true

The same snippet both times. Only the option changed.

With it off

"strictNullChecks": false

export function len(s: string | null) {
  return s.length
}

Compiles clean

With it on

"strictNullChecks": true

export function len(s: string | null) {
  return s.length
}

Emits TS18047

Why the compiler bothers

This is the single largest semantic difference between two TypeScript configurations, and it is why a type copied out of one codebase can mean something else in another. With the flag off the compiler has no way to express "definitely there", so every annotation is a half-promise. With it on, absence becomes a thing you write down — and every place the old code was lying about it turns into an error, which is why turning this on late is measured in weeks rather than an afternoon.

Takeaway

An annotation without this flag is a suggestion. With it, it is a claim.

Where to go next

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