Error: `Union types are not allowed in index signatures. Consider using a mapped object type instead. However, mapped object type cannot be passed as a string or number.`

Question

Error: `Union types are not allowed in index signatures. Consider using a mapped object type instead. However, mapped object type cannot be passed as a string or number.`

Encountering the error message

unions can't be used in index signatures, use a mapped object type instead

prompted me to try converting a string literal union (key names of an interface) into a mapped object. I found guidance on how to do this here: https://github.com/microsoft/TypeScript/issues/24220#issuecomment-390063153

Despite trying different approaches, I am still unable to get it working:

const returnObject: { 
   [index : mappedObjectOfConfigDataKeys] : Partial<ConfigData>
 } = {...accumulator};

// Error Message: TS1023: An index signature parameter type must be either 'string' or 'number'.

To address the issue, I attempted the following:

const returnObject: Record<keyof ConfigData, Partial<ConfigData>> = {...accumulator};

This resulted in the error:

Type '{ languageCode?: string | undefined; numberOfRepeats?: number | undefined; projectId?: string | undefined; }' is not assignable to type 'Record<"languageCode" | "numberOfRepeats" | "projectId", Partial<ConfigData>>'. 
Types of property 'languageCode' are incompatible.
Type 'string | undefined' is not assignable to type 'Partial<ConfigData>'.
Type 'undefined' is not assignable to type 'Partial<ConfigData>'.ts(2322)

Another attempt was made with the code:

const returnObject: Record<"languageCode" | "numberOfRepeats" | "projectId", Partial<ConfigData>> = {...accumulator};

Which resulted in a similar error:

Type '{ languageCode?: string | undefined; numberOfRepeats?: number | undefined; projectId?: string | undefined; }' is not assignable to type 'Record<"languageCode" | "numberOfRepeats" | "projectId", Partial<ConfigData>>'.
Types of property 'languageCode' are incompatible.
Type 'string | undefined' is not assignable to type 'Partial<ConfigData>'.
Type 'undefined' is not assignable to type 'Partial<ConfigData>'.ts(2322)

typescript index-signature

Answer 1

Answer №1

I'm uncertain of the goal you're aiming for:

const resultObject: Record<keyof ConfigurationData, Partial<ConfigurationData>> = {...accumulator};

Answer 2

I'm uncertain of the goal you're aiming for:

const resultObject: Record<keyof ConfigurationData, Partial<ConfigurationData>> = {...accumulator};

Answer 3

Answer №2

Instead of using an index signature for a mapped object, I opted to transform the value into a mapped type.

The original code snippet:

const returnObject: { 
   [index : mappedObjectOfConfigDataKeys] : Partial<ConfigData>
 } = {...accumulator};

can be simplified to this:

const returnObject: Partial<ConfigData> = accumulator;

Answer 4

Instead of using an index signature for a mapped object, I opted to transform the value into a mapped type.

The original code snippet:

const returnObject: { 
   [index : mappedObjectOfConfigDataKeys] : Partial<ConfigData>
 } = {...accumulator};

can be simplified to this:

const returnObject: Partial<ConfigData> = accumulator;

Error: `Union types are not allowed in index signatures. Consider using a mapped object type instead. However, mapped object type cannot be passed as a string or number.`

Answer №1

Answer №2

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