reduce array of objects into nested arrays

Question

I am trying to nest elements within each-other, but today I am having a brain fart.

If an element in the array is isParent: true, then it will create a new array within the exist array, and all following elements will be nested inside that array.

Here is what I have tried so far:

//input [{},{},{},{isParent}, {}, {}, {isParent}, {}, {}]
//desired output [{}, {}, {}, [{}, {}, {}, [{}, {}, {}]]]

var nestInParent = elements => {
      let ignoreIndexAfter = null;
      return elements.reduce((acc, { component, isParent }, index) => {
        if (isParent) {
          let remaining = elements.slice(index + 1);
   			ignoreIndexAfter = index;
          if (remaining.length > 0) {
            return [...acc, [component, ...nestInParent(remaining)]];
          } else {
            return [...acc, [component]];
          }
        } else {
			if(ignoreIndexAfter === null || index < ignoreIndexAfter){
               return [...acc, component];
            }
return acc;
        }
      }, []);
    };
const isParent = true;
const input = [
  {component:0},
  {component:1},
  {component:2},
  {isParent, component:3},
  {component:4},
  {component:5},
  {isParent, component:6},
  {component:7},
  {component:8}
];
const expected = [
  0,
  1,
  2,
  [
    3,
    4,
    5,
    [
      6,
      7,
      8
    ]
  ]
];
const output = nestInParent(input);
console.log("input:", input);
console.log("output:", output);
console.log("passes?", JSON.stringify(output) === JSON.stringify(expected));

.as-console-wrapper { max-height: 100% !important; }

I edited the code, that's no longer in there. it works a bit better, but not quite. — George
– George, Commented May 2, 2019 at 10:28
The main problem is that you traverse the whole array with every recursive step. That is a waste of time, and makes things overly complicated. Don't use reduce if not appropriate. — Jonas Wilms
– Jonas Wilms, Commented May 2, 2019 at 10:32

Nenad Vracar · Accepted Answer · 2019-05-02 10:38:51Z

You could also do this using reduce method and one variable to track if element with isParent prop is found.

const isParent = true;
const input = [{component:0},{component:1},{component:2},{isParent, component:3},{component:4},{component:5},{isParent, component:6},{component:7},{component:8}];

function nest(data) {
  let nested = false;
  return data.reduce((r, e, i) => {
    if(!nested) {
    	if(e.isParent) {
      	const res = nest(data.slice(i + 1))
        r.push([e.component, ...res])
        nested = true;
      } else {
        r.push(e.component)
      }
    }
    return r;
  }, [])
}

const result = nest(input);
console.log(result)

You can also write it like this.

const isParent = true;
const input = [{ component: 0 }, { component: 1 }, { component: 2 }, { isParent, component: 3 }, { component: 4 }, { component: 5 },{ isParent, component: 6 }, { component: 7 }, { component: 8 }];

function nest(data, nested) {
  return data.reduce((r, e, i) => {
    if(!nested) {
      if(e.isParent && i != 0) {
        r.push(nest(data.slice(i)))
        nested = true;
      } else {
        r.push(e.component)
      }
    }
    return r;
  }, [])
}

const result = nest(input);
console.log(result)

Fraction · Accepted Answer · 2019-05-02 12:15:22Z

I'd say that your code fails because it is overly complicated. You just need:

const result = [];
let current = result;

 for(const { component, isParent } of data) {
   if(isParent) {
     current.push(current = [component]);
   } else current.push(component);
 }

const input = [
  {component:0},
  {component:1},
  {component:2},
  {isParent: true, component:3},
  {component:4},
  {component:5},
  {isParent: true, component:6},
  {component:7},
  {component:8}
];

const result = [];
let current = result;

 for(const { component, isParent } of input) {
   if(isParent) {
     current.push(current = [component]);
   } else current.push(component);
 }

console.log(result);

Or if you really want to reduce, reduceRight:

const result = data.reduceRight((acc, { isParent, component }) => isParent ? [[component, ...acc]] : [component, ...acc], []);

const input = [
  {component:0},
  {component:1},
  {component:2},
  {isParent: true, component:3},
  {component:4},
  {component:5},
  {isParent: true, component:6},
  {component:7},
  {component:8}
];

const result = input.reduceRight((acc, { isParent, component }) =>
isParent ? [[component, ...acc]] : [component, ...acc], []);

console.log(result);

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