TypeScript reports error when using certain initial values in Array.reduce

Question

When using certain initial values in Array.reduce, TypeScript reports a strange error in this Playground example.

The error Argument of type 'number' is not assignable to parameter of type 'never' is shown for c in line 2 of the following example:

const foo = [1, 2, 3].reduce((p, c) => {
    p.bar.push(c);
    return p;
}, {bar: []});

document.write(`foo=${JSON.stringify(foo)}`);

The empty array accumulator is automatically typed as never. An alternate approach that should work would be to type accumulator p in the first line — frederj
– frederj, Commented Jan 17, 2020 at 16:45

Christian Ivicevic · Accepted Answer · 2019-01-13 18:37:26Z

1

This error occurs when you have strictNullChecks enabled and for a good reason. Your initial value for Array.reduce is {bar: []} which cannot be resolved to a safe type. Is the array any[] or number[]? Hence, TypeScript assumes it is never[] as it cannot infer the type of the array from the usage inside your lambda (it only works the other way round).

By the way, without strictNullChecks the array has type undefined[], but the flag basically replaces all occurenced of undefined as a type with never which causes your example to fail since and assignment of the form const foo = []; is of type never[] with strict settings.

In order to fix this issue you have to set your initial value to {bar: [] as number[]} to explicitly type the array and it works:

const foo = [1, 2, 3].reduce((p, c) => {
    p.bar.push(c);
    return p;
}, {bar: [] as number[]});

document.write(`foo=${JSON.stringify(foo)}`);

edited Jan 13, 2019 at 18:37

answered Jan 13, 2019 at 10:51

Christian Ivicevic

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9 Comments

doberkofler Over a year ago

Thank you for the explanation but I'm still very much confused. Given you explanation it is all about the untyped array but why does [1, 2, 3].reduce((p, c) => { p.push(c); return p; }, []); then work without error?

Christian Ivicevic Over a year ago

@doberkofler It does not work with strictNullChecks enabled either.

doberkofler Over a year ago

Correct. My mistake. So this case can actually simplified to the following example const foo = {foo: []}; foo.foo.push(1); that generates the same error. But why can i then use const bar = []; bar.push(1); without error?

Christian Ivicevic Over a year ago

Yet again bar is of type never[] and it does not compile for me with strictNullChecks enabled.

Christian Ivicevic Over a year ago

@doberkofler What the flag does is to replace occurences of the type undefined such as your arrays which are undefined[]. It replaces all occurences of undefined with never to cause compile time errors unless you explicitly type them.

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