The type definition for JSON.stringify erroneously has return type string

[mpiroc]

JSON.stringify currently has type:

stringify(value: any, replacer?: (this: any, key: string, value: any) => any, space?: string | number): string;
stringify(value: any, replacer?: (number | string)[] | null, space?: string | number): string;

For all inputs (including null) except undefined, this type is correct. However, JSON.stringify(undefined) returns undefined. Therefore, the return type of JSON.stringify should be changed to string | undefined.

Solution 1: Change return type for existing overloads

stringify(value: any, replacer?: (this: any, key: string, value: any) => any, space?: string | number): string | undefined;
stringify(value: any, replacer?: (number | string)[] | null, space?: string | number): string | undefined;

Solution 2 (untested): Only change the return type for undefined inputs

stringify(value: {} | null, replacer?: (this: any, key: string, value: any) => any, space?: string | number): string;
stringify(value: {} | null, replacer?: (number | string)[] | null, space?: string | number): string;
stringify(value: undefined, replacer?: (this: any, key: string, value: any) => any, space?: string | number): undefined;
stringify(value: undefined, replacer?: (number | string)[] | null, space?: string | number): undefined;

TypeScript Version: 3.6.3 and 3.7-Beta (on typescript playground), 3.6.4 (locally)

Search Terms:

• stringify
• JSON.stringify

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
const result = JSON.stringify(undefined)
console.log(result) // undefined
console.log(result.length) // Uncaught TypeError: Cannot read property 'length' of undefined

Expected behavior:
Code that assumes that the result of JSON.stringify is not undefined should fail to compile.

Actual behavior:
Code that assumes that the result of JSON.stringify is not undefined compiles, but fails at runtime.

Playground Link:
http://www.typescriptlang.org/play/?ssl=3&ssc=93&pln=3&pc=61#code/MYewdgzgLgBATgUwgVwDawLwwFIGUDyAcgHTRwCWYA5uQGYCeAFMmACYK2UKsCUAUKEghUCYqhBVGiFOh4wA9PJgt2nMNwHgIw0eMnS0UMQmpQAFnMUwAqmGABDZFTOwAKvQAOCAKJw4IOAAuGABhezAwEFhEe1YYD38vOCh6GAByEVMzNJgQWmU2Di5WIA

Related Issues:

[DanielRosenwasser]

Duplicate of #18879.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.