typescriptstrict-mode

Why filtering objects which has undefined key gives error in TypeScript strict mode

I have this piece of code written in TypeScript with strict mode enabled.

const arr: ({ name?: string })[] = [
    { name: 'foo' },
    {},
    { name: 'bar' }
]

function select(): string[] {
    return arr
        .filter(e => e.name !== undefined)
        .map(e => e.name)
}

It fails to compile with this error:

Type '(string | undefined)[]' is not assignable to type 'string[]'.
  Type 'string | undefined' is not assignable to type 'string'.
    Type 'undefined' is not assignable to type 'string'.

Why is that? Aren't all objects with property name is undefined filtered out before being mapped? If I change the return type of the select function to (string | undefined)[], it compiles successfully.

Solution

  • You filtered an array but that filter itself does not change returning type so that's why filter(e => e.name !== undefined) returns (string | undefined)[]. You should "cast" e.name to string to turn (string | undefined) into string:

    .map(e => e.name as string)