noImplicitReturns

Every path or none

A function where some branches return a value and others fall off the end is almost always a missing case.

Since: TypeScript 2.0
strict: Not in strict
In your tsconfig: "noImplicitReturns": true

The same snippet both times. Only the option changed.

With it off

"noImplicitReturns": false

export function grade(n: number) {
  if (n > 50) {
    return 'pass'
  }
}

Compiles clean

With it on

"noImplicitReturns": true

export function grade(n: number) {
  if (n > 50) {
    return 'pass'
  }
}

Emits TS7030

Why the compiler bothers

Without an annotation the compiler infers `string | undefined` and everything downstream inherits the hole, so the mistake is reported — if it is reported at all — somewhere far from where it was made. This flag reports it here. It is about the shape of the function rather than the type of the result, which is why it catches the case an explicit return type would also catch and the case where you never wrote one.

Takeaway

An exhaustive switch that returns from every case satisfies it without a final return.

Where to go next

22 options