The "if(x in obj)" statement in Typescript does not properly narrow down my custom Record

I am struggling with a code snippet where I am trying to check if a string exists in my custom record using the if(x in obj) guard statement, but it seems to not be working as expected. Below is the sample code snippet that is throwing an error:

type Answer = 'yes' | 'no';
type ToDo = Partial<Record<Answer, string>>;

function doFunction(toDo: ToDo, hisAnswer: string): string {
  if (hisAnswer in toDo) {
    return toDo[hisAnswer];
    // ^--- Error!
    // Element implicitly has an 'any' type because expression of type
    // 'string' can't be used to index type 'Partial<Record<Answer, string>>'.
    // No index signature with a parameter of type 'string'
    //  was found on type 'Partial<Record<Answer, string>>'.ts(7053)
  }
}
typescripttypeguards

Answer №1

While using TypeScript, it's important to understand that even when a variable conforms to a specific type, there may still be unexpected properties present. Take the following code snippet as an example:

const task = {
   yes: "hello",
   no : "world",
   foo: 42
};

const result = performAction(task, "foo");

console.log(typeof result); //number

Playground Link (fails compilation but can be run to see the result)

In this scenario, the variable task technically fits the structure of the Task type by having the keys yes and no. However, it also contains a key foo with a value that is not a string. This inconsistency is why TypeScript cannot guarantee the output as per the defined type - in theory, it should return a string. Yet, during runtime, the actual data passed into the variable task could vary widely, making it impossible for the compiler to predict all possible outcomes beforehand.

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