[Solved] Conditional types

I was reading the TS docs but I can't seem to grasp this concept very well. I have the following interface:

interface Response {
  hasErrored: boolean;
  body?: string;
}

interface Response {
  hasErrored: boolean;
  body?: string;
}

I want to let TypeScript know that my body is always available if "hasErrored" is false instead of checking if body exists in the first place. Thank you in advance!

3 Replies

oldben•2y ago

You are looking for a discriminated union type here.

type Response = { 
  hasErrored: false; } | {
  hasErrored: true;
  body: string;
}

type Response = { 
  hasErrored: false; } | {
  hasErrored: true;
  body: string;
}

seanOP•2y ago

Thank you so much, in two years of typescript I somewhat never had the need to use them hahaha

oldben•2y ago

Happy to help!

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[Solved] Conditional types