将可为空的对象值转换为Typescript中的字符串

Question

A little stuck trying to get the response from a GraphQL API typed properly so that I can use it with forms.

The reason I'm trying to do this is because React inputs expect values to be strings and not null. So I need to convert my nullable fields to empty strings before passing them on to the JSX.

This is a bit of a contrived case but should give the gist.

Interface IApiResult {
    title: string;
    description: string | null;
}

// I would expect this to have the shape { title: string, description: string }
//
type NonNullApiResult<T> = {
    [P in keyof T]: string
}

// API result
const result: IApiResult = { title: 'SO TS question', description: null }

// Mapped API result where all values must be strings
const mappedResult: NonNullApiResult<IApiResult> = {
    title: '',
    description: ''
}

// HERE: How can these be merged so that `mappedResult` stays
// of type NonNullApiResult and the data looks like:
//
// mappedResult = { title: 'SO TS question', 'description': '' }

I tried this..

// Loop through the result and convert null fields to empty strings
for (const key in result) {
    if (result.hasOwnProperty(key)) {

        // `value` is being given the type "any".
        // I would expect it to be "string | null"
        const value = result[key]

        // This passes. I'm guessing because `value` is "any"
        // However, it will still pass the null value into `mappedResult`
        // I would expect this to fail w/ "null not assignable to string"
        mappedResult[key] = value

        // This what I would expect to do
        // mappedResult[key] = value === null ? '' : value
    }
}

mappedResult is still of type NonNullApiResult<IApiResult> but if I console.log(mappedResult) I get the this in the browser:

{description: null, title: 'SO TS question'}

If I then do something in React like this it passes because it thinks description is a string

<input name="description" id="description" type="text" value={mappedResult.description} />

But in the console I get the expected error:

Warning: `value` prop on `input` should not be null. Consider using an empty string to clear the component or `undefined` for uncontrolled components.

Any help or suggestions are appreciated! This is using Typescript version 3.1.6

Answer 1

Try something like this:

function mapData(data){
  const mappedResult = {};
  for (const key in result) {
    if (result.hasOwnProperty(key)) {
      mappedResult[key] = result[key] || "";
    }
  }
  return mappedResult;
}

It should check each key in object, and if it is a falsy one (null || undefined) assign it an empty string.

And to clarify some more - what is i variable?