Array.reduce initialValue allows incorrect array type to be supplied

[jamesbrobb]

Bug Report

When supplying an array to the initialValue argument of Array.reduce and specifying the type of that array as a multi type array (i.e Array.reduce<(string|number|boolean)[]>), it's possible to then supply an array typed to only one of those types to the initialValue argument (i.e string[]) without an error being raised.

🔎 Search Terms

typescript array reduce initial value bug

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about reduce

⏯ Playground Link

Playground example

💻 Code

type arrayTypes = string | number | boolean;

const arr = [1, 2, 'three', 'four', 'five', 6, true],
    store: string[] = [];

const result = arr.reduce<(arrayTypes)[]>((accum: (arrayTypes)[], value: arrayTypes):  (arrayTypes)[] => {
    
    accum.unshift(value);
    return accum;

}, store);

console.log(result); // [true, 6, "five", "four", "three", 2, 1] 
console.log(store); // [true, 6, "five", "four", "three", 2, 1] 
console.log(result === store); // true

🙁 Actual behavior

Allows an incorrect array type to be supplied to initialValue argument

🙂 Expected behavior

Supplying the incorrect array type to InitialState argument should raise a type error

[jcalz]

This is working as intended. Method parameters are intentionally bivariant, which leads to Array<T> being covariant in T; hence string[] is considered to be a subtype of (string | number | boolean)[]. This is unsound, but convenient. Strict soundness is not a goal of TypeScript (see Non-Goal #3). See this comment on #274 and this comment on #9825 for more information.

[typescript-bot]

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