useUnknownInCatchVariables

What was thrown is not an Error

JavaScript can throw anything — a string, undefined, a number — so a caught value is `unknown` rather than `any`.

Since: TypeScript 4.4
strict: In strict
In your tsconfig: "useUnknownInCatchVariables": true
Compiled with: "strict": true

The same snippet both times. Only the option changed.

With it off

"useUnknownInCatchVariables": false

export function run() {
  try {
    throw new Error('boom')
  } catch (error) {
    return error.message
  }
}

Compiles clean

With it on

"useUnknownInCatchVariables": true

export function run() {
  try {
    throw new Error('boom')
  } catch (error) {
    return error.message
  }
}

Emits TS18046

Why the compiler bothers

`error.message` on a caught value is the single most common unchecked assumption in a typed codebase, and it is wrong far more often than it looks: a rejected promise carrying a string, a library throwing a plain object, a `null` from code that predates all of this. `unknown` forces the narrowing that was always required — `error instanceof Error` — and the cost is one line at each catch, once.

Takeaway

Narrow it once at the boundary, not at every use inside the block.

Where to go next

22 options