Debugging Errors: Unintuitive Error Message

Overview

In this example, we’ll look at a complex and unintuitive error message.

Below you can find a short piece of code using the RxJS library. In short, RxJS is a library for creating and manipulating streams of data. The piece of code below is supposed to create a stream of five numbers, map it into a stream of those numbers multiplied by two, and finally return a stream that emits only the last number.

const source = of(1, 2, 3, 4, 5).pipe(
  map(x => x * 2),
  last // 🔴 Error!
);

When compiled, this code will yield a large and complex error message.

Argument of type '{ <T, D = T>(predicate?: null | undefined, defaultValue?: D | undefined): OperatorFunction<T, T | D>; <T, S extends T>(predicate: (value: T, index: number, source: Observable<T>) => value is S,

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