Given:

let input = [0, 1, 2,,,,,7]

I want to get:

let output = [, 1, 22, 333, 4444, 55555, 666666, 7777777]

(i.e. value equal key times the key)

How can I map (or equivalent) input including empty values?
If I do input.map() I will get [, 1, 22,,,,,7777777] because map will -of course- not iterate empty values. Data can be arbitrary, strings, objects, numbers, etc. Using numbers in this example for simplicity.

Given:

let input = [0, 1, 2,,,,,7]

I want to get:

let output = [, 1, 22, 333, 4444, 55555, 666666, 7777777]

(i.e. value equal key times the key)

How can I map (or equivalent) input including empty values?
If I do input.map() I will get [, 1, 22,,,,,7777777] because map will -of course- not iterate empty values. Data can be arbitrary, strings, objects, numbers, etc. Using numbers in this example for simplicity.

• javascript
• arrays

Share Improve this question Follow edited Jul 27, 2020 at 4:34 user10539074 asked Jul 27, 2020 at 2:59 adelriosantiagoadelriosantiago 8,1441010 gold badges4545 silver badges7474 bronze badges 7

• Does it have to be input = [0, 1, 2,,,,,7] or it it possible to simply use Array.from({ length: 8 }, (_, index) => index) instead, in your use-case, or just Array.from({ length: 8 }, (_, index) => String(index).repeat(index)) to begin with? – Sebastian Simon Commented Jul 27, 2020 at 3:00
• “Data can be arbitrary, strings, objects, numbers, etc.” — so what’s the algorithm for “arbitrary” data? You assume that integers can be filled in in the missing spots. What’s the expected result for [1, 2, 3, , , 99]? What’s the expected result for [1, 2, "a", 99]? – Sebastian Simon Commented Jul 27, 2020 at 3:07
• @user4642212 The problem is not the map result. But rather how to "map" (or whatever) empty values. For example [1,,1] could yield to [{...}, {...}, {...}]. – adelriosantiago Commented Jul 27, 2020 at 3:14
• So you want arr.fill().map(...)? – Unmitigated Commented Jul 27, 2020 at 3:15
• i would suggest something like this [0, 1, 2, 3, 4, 5].map((el, index) => Array(el).fill(null).map(() => index).join("")) – Teiem Commented Jul 27, 2020 at 3:21

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3 Answers 3

Sorted by: Reset to default 7

You can fill the array before applying a mapping function so that all indexes are iterated over, including empty ones.

let result = input.fill().map((_,index)=>{/*...*/});

The mapping function provided as the second argument to Array.from can be similarly used.

let result = Array.from({length: input.length}, (_, index)=>{/*...*/});

Spread syntax does not ignore empty slots, so we can create a copy of the array with spread syntax and map over that. This has an added advantage of having the elements at each index available as well.

let result = [...input].map((elem, idx)=>{/*...*/});

map() can't include empty values. w3schools

map() does not execute the function for array elements without values.

So, either iterate the array with a for-loop:

out = [];
for (var i = 0; i < input.length; i++) {
    var item = input[i];
    if (item != null)
        out[i] = item.toString().repeat(item);
}

Or create another mapx() functor:

Array.prototype.mapx = function(fn) {
    var out = [];
    for (var i = 0; i < this.length; i++) {
        var item = this[i];
        // if (item == null) item = sherlockHolmesKnow();
        out[i] = fn(item, i);
    }
    return out;
};

and apply on the input:

input.mapx( (val, index) => (val == null ? null : (val+'').repeat(val)) )

which yields ["", "1", "22", null, null, null, null, "7777777"].

Let's skip the AI part of clever guessing the numbers in the empty slots.

Here is another approach too verbose but it works, with for loop you can loop through null values

arr=[0, 1, 2,,,,,7]
res=[]
f=false
for(let i=0;i<arr.length-1;i++){
  if(arr[i]!=undefined){
      res.push(Number(String(arr[i]).repeat(arr[i])))
      //f=false
  }
  if(arr[i]==undefined&&f==false){
    n= arr[i-1]  
    f=true
    res.push(Number(String(n+1).repeat(n+1)))}
    if(f==true && arr[i]==undefined){
     n=Number(res[res.length-1].toString().charAt(0))+1
      res.push(Number(String(n).repeat(n)))
    }
}
console.log(res)