The limitation of accessing an object using a key from a callback in Typescript

Question

The limitation of accessing an object using a key from a callback in Typescript

Whenever I run the code below, I encounter an error message that states

Element implicitly have any type because the expression of type string can't be used to index type Chapter

. Can someone help me understand what this means?

The main goal of this code is to update a specific value using the key from the callback:

let chaptersWithStrings = props.chapters;
        chaptersWithStrings.forEach((chapter)  => {

            Object.entries(chapter).forEach(([key, value]) => {
                if (typeof value === 'object') {
                    chapter[key] = JSON.stringify(value)
                }
            })

});

This is how the chapter interface looks like:

export interface Chapter {
    id: string,
    code: string,
    number: number,
    policies: Policy | string,
}

I would really appreciate any assistance provided.

Thank you.

typescript

Answer 1

Answer №1

Extracting properties from chapter object using Object.entries()

return string[]

When iterating through keys in a foreach loop, ensure the key is of type string to avoid TypeScript error.

To resolve this issue, there are 2 possible solutions:

Add index type signature to your interface:

export interface Chapter {
    [key: string]: any;
    id: string;
    code: string;
    number: number;
    policies: Policy | string;
}

For more information on index signatures

Redefine your key:

Object.entries(chapter).forEach(([key, value]) => {
   const objectKey = key as keyof typeof chapter;            
   if (typeof value === 'object') {
       chapter[objectKey] = JSON.stringify(value)
   }
})

Explanation on why object.keys may return string[] instead of inferer keyof object type: here is the answer

edit based on the comment:

export interface IChapter {
  id: string;
  code: string;
  number: number;
}

const chapter: IChapter = {
  id: '',
  code: '',
  number: 1,
};

Object.entries(chapter).forEach(([key, value]) => {
  const objectKey = key as keyof typeof chapter;
  if (typeof value === 'object') {
    switch (objectKey) {
      case 'id':
      case 'code':
        chapter[objectKey] = JSON.stringify(value);
        break;
      case 'number':
        chapter[objectKey] = 0; // ! ! here has to be number not JSON.stringify(value) since this is string and you can not assign string to type number
    }
  }
});

Answer 2

Extracting properties from chapter object using Object.entries()

return string[]

When iterating through keys in a foreach loop, ensure the key is of type string to avoid TypeScript error.

To resolve this issue, there are 2 possible solutions:

Add index type signature to your interface:

export interface Chapter {
    [key: string]: any;
    id: string;
    code: string;
    number: number;
    policies: Policy | string;
}

For more information on index signatures

Redefine your key:

Object.entries(chapter).forEach(([key, value]) => {
   const objectKey = key as keyof typeof chapter;            
   if (typeof value === 'object') {
       chapter[objectKey] = JSON.stringify(value)
   }
})

Explanation on why object.keys may return string[] instead of inferer keyof object type: here is the answer

edit based on the comment:

export interface IChapter {
  id: string;
  code: string;
  number: number;
}

const chapter: IChapter = {
  id: '',
  code: '',
  number: 1,
};

Object.entries(chapter).forEach(([key, value]) => {
  const objectKey = key as keyof typeof chapter;
  if (typeof value === 'object') {
    switch (objectKey) {
      case 'id':
      case 'code':
        chapter[objectKey] = JSON.stringify(value);
        break;
      case 'number':
        chapter[objectKey] = 0; // ! ! here has to be number not JSON.stringify(value) since this is string and you can not assign string to type number
    }
  }
});

Answer 3

Answer №2

Check out the following code snippet for assistance:

Iterate through each key in the 'chapter' object and update specific values when necessary:
Object.keys(chapter).forEach((key: string) => {
  if (chapter[key] !== null && typeof chapter[key]=== 'object') {
    chapter[key] = ...; // Update the value corresponding to the key
  }
});

Answer 4

Check out the following code snippet for assistance:

Iterate through each key in the 'chapter' object and update specific values when necessary:
Object.keys(chapter).forEach((key: string) => {
  if (chapter[key] !== null && typeof chapter[key]=== 'object') {
    chapter[key] = ...; // Update the value corresponding to the key
  }
});

The limitation of accessing an object using a key from a callback in Typescript

Answer №1

Answer №2

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