What is causing the Typescript type checker to become frustrated with this unassigned type?

I am dealing with a React component that has certain props that need to be serialized and stored, as well as a function contained within them.

  • However, my storage utility does not support storing functions, and I do not have the requirement of storing them either. Hence, I wrote a function to create a copy of the props object without including the function component:
/**
 * Removes functions from the components props, so that the props can be pouched
 */
strippedProps(): Object {
    let strippedProps: Object = {};

    Object.keys(this.props).forEach((key) => {
        let prop: any = this.props[key];

        if (typeof (prop) != 'function') {
            strippedProps[key] = prop;
        }
    })

    return strippedProps;
}

I am facing TypeScript errors, and it is confusing because the variable prop has been explicitly defined as any.

https://i.sstatic.net/oZO5p.png

Why is TypeScript showing dissatisfaction with the initial error message? How can I modify the code to satisfy the compiler while still achieving the goal of generating this dynamic object?

Answer №1

The specified index signature is missing from the type of this.props.


Indexable Types

In a similar manner to how interfaces can be used to define function types, they can also be used to define types that can be accessed by index like a[10] or ageMap["daniel"]. Indexable types have an index signature that defines the allowable types for indexing into the object and the corresponding return types. Here's an example:

To address this issue, you must specify in Typescript that the object type of this.props has keys of type string:

interface QuestionViewProps { 
   ... your definition,
   [key: string]: myType,
}

An alternative approach is to suppress the warning using "suppressImplicitAnyIndexErrors": true.


A third option would be to cast to any, although this method would sacrifice type safety:

 let prop: any = (<any>this.props)[key];

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