Error occurs when using JSON.stringify with a Typescript array map

Question

Error occurs when using JSON.stringify with a Typescript array map

When running this code snippet in Typescript:

[].map(JSON.stringify);

An error is being thrown:

Argument of type '{ (value: any, replacer?: ((key: string, value: any) => any) | undefined, space?: string | number | undefined): string; (value: any, replacer?: (string | number)[] | null | undefined, space?: string | ... 1 more ... | undefined): string; }' is not assignable to parameter of type '(value: never, index: number, array: never[]) => string'. Types of parameters 'replacer' and 'index' are incompatible. Type 'number' is not assignable to type '((key: string, value: any) => any) | undefined'.

This seems like a potential bug with Typescript. However, I would appreciate it if someone could verify whether I am making a mistake before I raise an issue on GitHub.

Current Typescript version being used: 3.0.3

typescript

Answer 1

Answer №1

The compatibility issue between the signature of JSON.stringify and Array.map arises from their differing second parameters:

interface JSON {
    /**
      * The function converts a JavaScript value to a JSON string.
      * @param value A JavaScript value, such as an object or array, to convert.
      * @param replacer A function that modifies the results.
      * @param space Adds formatting for better readability in the resulting JSON text.
      */
     stringify(value: any, replacer?: (key: string, value: any) => any, space?: string | number): string;
}

interface Array<U> {
    /**
      * It executes a custom callback on each element of an array and returns an array with the new values.
      * @param callbackfn A function that takes up to three arguments. It is called once for each array element.
      * @param thisArg An optional object for the callback function's 'this' reference. If omitted, undefined is used.
      */
    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];
}

This discrepancy highlights the mismatch between the signatures of stringify and callbackfn due to the inconsistent nature of their second arguments.

However, you can still use

[].map(i => JSON.stringify(i))

without any issues.

Answer 2

The compatibility issue between the signature of JSON.stringify and Array.map arises from their differing second parameters:

interface JSON {
    /**
      * The function converts a JavaScript value to a JSON string.
      * @param value A JavaScript value, such as an object or array, to convert.
      * @param replacer A function that modifies the results.
      * @param space Adds formatting for better readability in the resulting JSON text.
      */
     stringify(value: any, replacer?: (key: string, value: any) => any, space?: string | number): string;
}

interface Array<U> {
    /**
      * It executes a custom callback on each element of an array and returns an array with the new values.
      * @param callbackfn A function that takes up to three arguments. It is called once for each array element.
      * @param thisArg An optional object for the callback function's 'this' reference. If omitted, undefined is used.
      */
    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];
}

This discrepancy highlights the mismatch between the signatures of stringify and callbackfn due to the inconsistent nature of their second arguments.

However, you can still use

[].map(i => JSON.stringify(i))

without any issues.

Answer 3

Answer №2

Don't worry, it's not a glitch. The message is simply indicating that the data type for the second parameter in JSON.stringify (the replacer function) doesn't align with the expected second parameter type number (representing the index) in the map function.

Answer 4

Don't worry, it's not a glitch. The message is simply indicating that the data type for the second parameter in JSON.stringify (the replacer function) doesn't align with the expected second parameter type number (representing the index) in the map function.

Error occurs when using JSON.stringify with a Typescript array map

Answer №1

Answer №2

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