I need a set of predefined keys, but users should not be restricted to only using those keys.
type B = {
a: string;
b: number;
}
type T = keyof B | string;
function someFunc(key: T) {}
someFunc(); // key type is `T`
In the scenario above, I am looking for autocomplete suggestions for a and b, while still allowing users to input any other string. Is this achievable?
When the compiler condenses the union "a" | "b" | string down to just string, it may seem correct in terms of type acceptance since any string value is allowed, rendering "a" and "b" as specific instances. However, this reduction erases some information that was originally present in the "a" | "b" | string type, particularly impacting IntelliSense and IDE completion suggestions.
As of now, TypeScript does not possess a built-in mechanism to address this issue. Several problems related to this have been reported on GitHub, with prominent ones like microsoft/TypeScript#29729 being actively reviewed by the design team (as of 2020-05-23). Additional concerns can be found under issues such as microsoft/TypeScript#26277, microsoft/TypeScript#33471, and microsoft/TypeScript#34714. Hence, current language limitations necessitate the utilization of workarounds for similar functionalities.
A common workaround involves defining a type that allows the acceptance of any string value without collapsing it into just string. For instance:
function someFunc<S extends string>(key: S | keyof B) { }
In this example, someFunc() is generic in S, which must abide by the string constraint. The key parameter holds the type S | keyof B. Regardless of the string input provided for key, it gets inferred as the type for
S</code, thereby allowing any <code>stringsomeFunc("a"); // accepted
someFunc("b"); // approved
someFunc("omega"); // acknowledged
Despite these capabilities, autocomplete features will recommend "a" and "b" due to uncertainties surrounding S during suggestion generation.
Successfully implementing such workarounds involves handling potential edge cases arising from transforming a concrete type into a generic one. It's crucial to carefully evaluate whether the resulting side effects are acceptable considering this approach is merely a workaround.
In conclusion, effective leverage of these techniques can enhance your TypeScript development experience. Best of luck!
type X = {
name: string;
age: number;
}
type Y = {[key:string]: string | number } & X
const y: Y = {
name: "John",
age: 30,
city: "New York"
}
Experience seamless auto completion with this link
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