如何在 Array.reduce() 中返回 null？

Question

I have an array which could looks like this. It's an array of dynamic length containing either a string or object.

['h', 'e', {apple: 2}, 'l', 'p']

I need to turn it into this:

 [null, null, 'apple', 'apple', null, null]

So I was trying to use reduce method and started with turning strings into null.

.reduce(
      (acc: Array<null | string>, val) => {
        if (typeof val === "string") return acc.concat(null);
        
        return acc.concat(val)
      },
      []
    );

it returns back:

[null]

and I expected:

[null, null, {apple: 2}, null, null]

Performance is moderately important, is it possible to do something like this with looping over array just once without resulting to for loops?

EDIT: sorry I made a stupid mistake, seems like my library turns it into ["he", {apple:3}, "lp"].

Answer 1

You can use Array.prototype.flatMap -

 const process = (t = []) => t.flatMap ( v => Object(v) === v? Object.entries(v).flatMap(([ s, n ]) => Array(n).fill(s)): [ null ] ) const data = ['h', 'e', {apple: 2}, 'l', 'p'] const result = process(data) console.log(JSON.stringify(result)) // [null,null,"apple","apple",null,null]

If the object has multiple keys, each item will be expanded -

const result =
  process(['h', 'e', {apple: 2, pear: 3}, 'l', 'p', {berry: 1}])
  
console.log(JSON.stringify(result))
// [null,null,"apple","apple","pear","pear","pear",null,null,"berry"]

---

Array.prototype.flatMap is just a specialised version of Array.prototype.reduce -

Array.prototype.flatMap = function (f, context)
{ return this.reduce
    ( (r, x) => r.concat(f.call(context, x))
    , []
    )
}

Answer 2

that?

 const arr = ['h', 'e', {apple: 2}, 'l', 'p'] const rep = arr.reduce((a,c)=> { if (c instanceof Object) { let pair = Object.entries(c)[0] for (let n=0;n<pair[1];++n) a.push(pair[0]) } else a.push(null) return a },[]) console.log(...rep )