Advanced TypeScript Masterclass/

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Debugging Errors: Narrowing Types

Debugging Errors: Narrowing Types

This lesson walks through the debugging of a compile error that can be addressed by appropriate type narrowing.

We'll cover the following...

Overview #

Another error that we’ll look at is an example of a situation when TypeScript is not able to narrow the type of a variable, even though it might be obvious from the perspective of the programmer.

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interface Person { name: string; employer?: string; };
function getEmployers(persons: Person[]): string[] {
    return persons
        .filter(person => person.employer !== undefined)
        .map(person => person.employer); // 🔴 Error! 
}

The above code will result in the following error message.

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'.ts(2322)

Digging deeper

Let’s break down the error message.

Type
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