Adding types to a recursive function

Question

I've this below function to flatten an array item,

 private flatArray(data): [] {
    return data.reduce((accu, val) => (Array.isArray(val) ? accu.concat(this.flatArray(val)) : accu.concat(val)), []);
  }

Now I wanted to add types to the argument, Below is how I did it,

  private flatArray<T>(data: Array<Array<T>> | Array<T>): Array<T>

But I get this error,

Cannot invoke an expression whose type lacks a call signature. Type '{ (callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T; (callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T; (callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U;...' has no compatible call signatures.ts(2349)

Any idea how to fix this issue ?

There is .flat() though ...

Jonas Wilms
– Jonas Wilms

2019-07-30 09:46:45 +00:00
Commented Jul 30, 2019 at 9:46 — Jonas Wilms
– Jonas Wilms, Commented Jul 30, 2019 at 9:46
I can use that, but it dosen't support IE

user007
– user007

2019-07-30 10:01:20 +00:00
Commented Jul 30, 2019 at 10:01 — user007
– user007, Commented Jul 30, 2019 at 10:01

GOTO 0 · Accepted Answer · 2019-07-30 10:38:29Z

2

You probably want to do

private flatArray<T>(data: Array<T | Array<T>>): Array<T>

A more robust approach in order to allow recursively nested arrays would be using a dedicated readonly type.

interface NestedArray<T> extends ReadonlyArray<NestedArray<T> | T> { }

// ...

private flatArray<T>(data: NestedArray<T>): Array<T>

Playground link

edited Jul 30, 2019 at 10:38

answered Jul 30, 2019 at 10:03

GOTO 0

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8 Comments

zerkms Over a year ago

Is there a way to do that without narrowing an array type to readonly?

GOTO 0 Over a year ago

You mean NestedArray<T> extends Array<NestedArray<T> | T>? Yes, that restricts the function to only accepting writable arrays.

Jonas Wilms Over a year ago

Are you sure that this solves the problem described by the OP? Cause I don't see any difference between Array<T> | Array<Array<T>> and Array<Array<T> | T>

zerkms Over a year ago

@JonasWilms one is array of T OR array of arrays of T; the other is Array whose elements can be either T or arrays of T simultaneously.

Jonas Wilms Over a year ago

@zerkms sure, but the OP might wants the first. Not sure how this addresses the question. Also not sure how this is related to the data.reduce(...) type missmatch.

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