TS18046

It is unknown

'value' is of type 'unknown'.

The compiler’s own words. Not translated — this is the string you pasted into a search box.

unknown is the type that refuses to be used until you prove what it is. That refusal is the entire feature.

Reproduction

declare const value: unknown

value.trim()

The build asserts this emits exactly this code.

Why the compiler says this

any says "stop checking"; unknown says "check first". They sit at the same place in the hierarchy — everything is assignable to both — but unknown is assignable to nothing without a narrowing step. This error is not an obstacle to route around with a cast: it is the type asking the one question a value from outside your program should always be asked.

Fixes

01
```
declare const value: unknown

if (typeof value === 'string') {
  value.trim()
}
```
Narrow it. Inside the guard the compiler knows it is a string, and the method call is checked like any other.

function isString(input: unknown): input is string {
  return typeof input === 'string'
}

declare const value: unknown

if (isString(value)) {
  value.trim()
}

Or write the check once as a type predicate and reuse it. The input is string return type is what turns a boolean function into something the compiler will narrow on.

Takeaway

Reach for unknown at every boundary — parsed JSON, a caught error, anything from the network — and let this error force the check you would have skipped.

Errors