TypeScript Array<>.reduce() Types

Question

Is there a way within TypeScript to change the type expected to be returned by an Array.reduce call from the type given to it? The code I'm having an issue with is below for the sake of an example:

let executionSequence: RelationalReversionQuery[][] = this.reduce(
    (accumulator: RelationalReversionQuery|RelationalReversionQuery[][], item: RelationalReversionQuery) => {
    if (accumulator instanceof RelationalReversionQuery) {
        if ((accumulator.ExecutionSequence === RelationalReversionQueryExecutionSequence.Indifferent) && (item.ExecutionSequence === RelationalReversionQueryExecutionSequence.Indifferent)) {
            return [[accumulator, item]]
        }
        return [[accumulator], [item]];
    }
    if (item.ExecutionSequence !== RelationalReversionQueryExecutionSequence.Indifferent) {
        accumulator.push([item]);
        return accumulator;
    }
    if (accumulator.length > 0) {
        let lastArray = accumulator[accumulator.length - 1];
        if (lastArray.length > 0) {
            if (lastArray[lastArray.length - 1].ExecutionSequence === RelationalReversionQueryExecutionSequence.Indifferent) {
                lastArray.push(item);
            } else {
                accumulator.push([item]);
            }
        } else {
            lastArray.push(item);
        }
    } else {
        accumulator.push([item]);
    }
    return accumulator;
});

this is extending the type Array<RelationalReversionQuery>.

What exactly is the issue? What do you expect happens and what happens instead? — VLAZ
– VLAZ, Commented Oct 5, 2021 at 5:30
@VLAZ Code-wise it works without the type issues in JavaScript, but in TypeScript I'm getting type errors in WebStorm. I'm expecting to take in an Array<RelationalReversionQuery> and output an Array<Array<RelationalReversionQuery>> - that is, there's a type change which occurs within the accumulator. — CoryG
– CoryG, Commented Oct 5, 2021 at 5:43

tmhao2005 · Accepted Answer · 2021-10-05 06:37:28Z

2

This is interesting. The built-in type doesn't allow you to return a different type of the original type unless you specify the 2nd argument of .reduce function in this case.

this.reduce((accumulator: RelationalReversionQuery|RelationalReversionQuery[][], item: RelationalReversionQuery) => {
  // ...
}, []);

//  ^ this is what you need to add

answered Oct 5, 2021 at 6:37

tmhao2005

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