strict

The whole family at once

Not an option so much as a subscription: it turns on eight other checks, and keeps turning on the ones added after you wrote your tsconfig.

Since: TypeScript 2.3
In your tsconfig: "strict": true

The same snippet both times. Only the option changed.

With it off

"strict": false

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

Compiles clean

With it on

"strict": true

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

Emits TS18047

Why the compiler bothers

Every flag under this one exists because some category of runtime failure has a compile-time shadow. Turning them on individually is how a codebase ends up with seven of the eight, missing the one that mattered. The subscription behaviour is the feature, not a hazard: a release that adds a new check adds it to your build, which is the only way a strictness setting can keep meaning the same thing as the language changes underneath it.

Takeaway

Turn it on first and switch off what you cannot afford yet, rather than the reverse.

Where to go next

22 options