TS18047

Value is possibly null

'value' is possibly 'null'.

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

A nullable value is used as though the `null` branch had already been ruled out.

Reproduction

function upper(value: string | null) {
  return value.toUpperCase()
}

The build asserts this emits exactly this code.

Why the compiler says this

With strict null checking, `string | null` is two real possibilities. Methods of `string` are safe only after control flow proves that the current value belongs to the string branch.

Fixes

function upper(value: string | null) {
  if (value === null) return ''
  return value.toUpperCase()
}

Guard the null case and choose the behavior your domain requires.

function upper(value: string | null) {
  return value?.toUpperCase() ?? ''
}

Use optional chaining with an explicit fallback for a compact transformation.

Takeaway

Narrow nullable values where you use them; a non-null assertion merely moves the risk to runtime.

Where to go next

Conditional types Try it in the Lab

Errors