There are certain features missing in TypeScript that are essential for ensuring the code works properly and the compiler can verify its type safety. One key missing feature is the ability to perform user-defined type assertions, as proposed in this GitHub issue: user-defined type assertions. This would allow specifying that the method this.readText() will eventually narrow the type of this to this & {text: string}. Additionally, it would be necessary to return a Promise of the type assertion instead of the assertion itself, requiring something like the propagation of type predicates, as suggested in this GitHub issue: propagation of type predicates. Despite these improvements, an implementation like the following does not compile:
// DOES NOT COMPILE, DON'T TRY THIS
async readText(): Promise<this as (this & {text: string})> {
this.text = await this.response.text();
}
This code attempts to return a promise that narrows the type of this to ensure the text property is definitely a string. Unfortunately, this approach does not work as intended.
To address this issue without altering the code at runtime, one can utilize type assertions, such as the non-null assertion operator, as demonstrated in @Phillip's response. Alternatively, restructuring the code to separate asynchronous and synchronous operations can ensure a more robust design:
// Example restructuring of code
type MyResponse = {
text: () => Promise<string>;
};
class AsyncResponseVerifier {
constructor(public response: MyResponse) {}
async readText(): Promise<SyncResponseVerifier> {
return new SyncResponseVerifier(await this.response.text());
}
}
class SyncResponseVerifier {
constructor(public text: string) {}
verifyTextContains(value: string) {
return this.text.includes(value);
}
}
In this revised setup, asynchronous operations are confined to one class, while synchronous operations are handled separately. By adopting this approach, the code becomes more organized and easier to manage:
// Implementing the revised structure
async function doThings() {
const asyncRV = new AsyncResponseVerifier({
text: async () =>
"an important forum raising awareness about serious things"
});
const syncRV = await asyncRV.readText();
syncRV.verifyTextContains("rum raisin");
}
By structuring the code in this manner, developers can avoid the confusion of mixing asynchronous and synchronous functionality within the same class. This separation improves code clarity and maintainability. Best of luck!