strictPropertyInitialization

A declared field has to be assigned

Declaring `token: string` says the property is a string. Before this flag, nothing checked that it ever became one.

Since: TypeScript 2.7
strict: In strict
In your tsconfig: "strictPropertyInitialization": true
Compiled with: "strict": true

The same snippet both times. Only the option changed.

With it off

"strictPropertyInitialization": false

export class Session {
  token: string
}

Compiles clean

With it on

"strictPropertyInitialization": true

export class Session {
  token: string
}

Emits TS2564

Why the compiler bothers

A class body is the one place in TypeScript where a type annotation and an assignment are written apart from each other, and the gap between them is where an `undefined` lives that the type denies. The flag closes it by requiring an initialiser, a constructor assignment, or the definite assignment assertion `token!: string` — which is you taking responsibility rather than the compiler taking your word for it. It needs `strictNullChecks`, because without it the property being `undefined` is not a contradiction in the first place.

Takeaway

`!` is not a fix. It is a note saying you checked and the compiler could not.

Where to go next

22 options