Is there a way to avoid using ts-ignore when using object properties referenced as strings in TypeScript? For example, if I have something like:
const myList = ['aaa', 'bbb', 'ccc'];
const appContext = {};
for (let i=0; i<3; i++) {
appContext[myList[i]] = 'newval' + i;
}
Even if I define my TypeScript type as:
interface IAppContext {
aaa: string;
bbb: string;
ccc: string;
I still encounter TypeScript errors. How can this be handled correctly?
It would be greatly beneficial if the example code provided was a genuine minimum reproducible example showcasing the exact errors without any other mistakes or issues. However, I have an understanding of what may be occurring.
The primary issue that you are likely to encounter is the inference by the compiler that myList is of type string[]. This inference is reasonable on the part of the compiler because individuals often create arrays like const foo = ["a","b"] with the intention of modifying the values within the array later (e.g., foo.push("c")). A const declaration in this context only restricts reassignment (foo = ...) and does not prevent setting properties or invoking state-modifying methods on it.
To assist the compiler in cases where incorrect inference occurs on a literal value such as ["aaa","bbb","ccc"], one can utilize a const assertion. By asserting
const myList = ['aaa','bbb','ccc'] as constreadonly ["aaa", "bbb", "ccc"] to myList. This type represents a tuple of length 3 with immutable values that are string literals "aaa", "bbb", and "ccc" in sequential order.
Subsequently, your code should mostly compile smoothly by ensuring that appContext is asserted as an IAppContext, for instance,
const appContext = {} as IAppContextconst appContext: IAppContext = {}IAppContext. Through a type assertion, you reassure the compiler that although appContext initiates as an invalid IAppContext, it will eventually become one after execution. Yet, it remains your responsibility to ensure proper initialization. For instance, if myList were just ['aaa','bbb'], the code would compile without warnings, but issues might arise at runtime when calling appContext.ccc.toUpperCase().
In conclusion, below is the revised error-free code:
interface IAppContext {
aaa: string;
bbb: string
ccc: string
}
const myList = ['aaa', 'bbb', 'ccc'] as const; // Constant context to retain literals
const appContext = {} as IAppContext; // Assertion due to initial emptiness
for (let i = 0; i < 3; i++) {
appContext[myList[i]] = 'newval' + i
}
I hope this explanation proves helpful. Best of luck!
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