Error TS2322: Type 'boolean' cannot be assigned to type 'undefined'. What is the best approach for dynamically assigning optional properties?

I am currently working on defining an interface named ParsedArguments to assign properties to an object, and here is what it looks like:

import {Rules} from "../Rules/types/Rules";

export interface ParsedArguments {
    //other props
    //...
    rules?: Rules,
}

In another function, I am attempting to create an object that retrieves values from cli and generates an object that complies with this interface. Here's a snippet of how I'm trying to achieve that:

private parseCliArgs(args: string[]): ParsedArguments {
        let res: ParsedArguments = {}

        for (const arg of args) {
            if (arg.startsWith('--')) {
                const {key, value} = this.getArgValues(arg)

                if (!res.rules) {
                    res.rules = {}
                }

                const rule: keyof Rules = key

                if (res.rules[rule]) {
                    //HERE I GET THE ERROR
                    //TS2322: Type 'STRING' is not assignable to type 'undefined'.
                    res.rules[rule] = value
                }
            }
        }

        return res
    }

Even after checking if the property of Rules exists, I still encounter the same error. I haven't been able to resolve it yet and would really appreciate your insights on this issue. This is how the Rules interface is structured:

export interface Rules {
    someProp1?: boolean,
    someProp2?: number,
    //...
}

I also experimented with using const rule = key as keyof Rules instead of const rule: keyof Rules = key, but it didn't make any difference.

If you require further clarification or if I missed any crucial information, please let me know. Thank you in advance.

Answer №1

The types in Rules and value are not compatible. In Rules, you have optional boolean and number (which implicitly includes undefined), whereas in value, you always have a string.
If you convert all props in Rules to strings, they will become compatible.

For a visual representation of what I mean, check out this playground

Answer №2

Here is my approach to the problem. I utilized Typescript generics within the built-in functionality, which proved to be quite effective in this scenario. By avoiding the need to define different types repeatedly, we can prevent a cascade of types from emerging. This allows for more dynamic results without unnecessary complexity. Take a look at the code snippet below:

let rules: Partial<Record<keyof Rules, string>> = {}

for (const arg of args) {
    if (arg.startsWith('--')) {
        const {key, value} = this.getArgValues(arg)

        if (this.isRuleKey(key)) {
            rules[key] = value
        }
    }
}

With this implementation, there's no need to create an additional version of the Rules type. Everything functions smoothly and efficiently.

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