Enumerating sliced array using original indices

user_172637 0
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I need to slice an array and enumerate the values but I also need the reference to the original index because I'm doing an async operation that needs to be mapped back to the original index in the original array when complete.

const array = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge'];
const slicedArray = array.slice(0, 3).map((v, i) => ({ v, i }));
// Returns: [{ "v": "foo", "i": 0 }, { "v": "bar", "i": 1 }, { "v": "baz", "i": 2 }]
// Required: [{ "v": "foo", "i": 0 }, { "v": "bar", "i": 1 }, { "v": "baz", "i": 2 }]
const slicedArray2 = array.slice(3, 6).map((v, i) => ({ v, i }));
// Returns: [{ "v": "qux", "i": 0 }, { "v": "quux", "i": 1 }, { "v": "corge", "i": 2 }]
// Required: [{ "v": "qux", "i": 3 }, { "v": "quux", "i": 4 }, { "v": "corge", "i": 5 }]

How can I achieve this?

slice arrays javascript
3 Answers

The items in the slice always have indexes that are exactly startIndex less than in the original array:

slice(0, 3) -> [0, 1, 2] -> [0 - 0, 1 - 0, 2 - 0]
slice(3, 6) -> [0, 1, 2] -> [3 - 3, 4 - 3, 5 - 3]
slice(n, …) -> [0, 1, …] -> [n + 0 - n, n + 1 - n, n + 2 - n, …]

So, just adding startIndex back should do the trick:

/**
 * @template Item
 * @param {Item[]} items
 * @param {number} startIndex
 * @param {number} [endIndex]
 * @returns {Array<{ item: Item; index: number }>}
 */
function slice(items, startIndex, endIndex = items.length) {
    return items.slice(startIndex, endIndex).map((item, index) => ({
        item,
        index: index + startIndex,
    }));
}


function slice(items, startIndex, endIndex = items.length) {
  return items.slice(startIndex, endIndex).map((item, index) => ({
    item,
      index: index + startIndex,
  }));
}

const letters = [ ...'abcdefghijklmnopqrstuvwxyz' ];

console.log(slice(letters, 0, 3));
console.log(slice(letters, 3, 6));
console.log(slice(letters, 6, 9));

user_45548830

Using array.map() You can get the all array value and index ! After this use .slice()

Try this code it's help you

 const array = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge'];
 const slicedArray2 = array.map((v, i) => ({ v, i })).slice(3, 6);
 console.log(slicedArray2, 'slicedArray2');
user_102730130

I actually like the accepted answer better, but for the record another solution without the slice

const data = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge'];

const slice = (items, startIndex, endIndex = items.length) => {
  return items.reduce((acc, v, i) => {
    if (i >= startIndex && i < endIndex )
      acc.push({ v, i })
    return acc;
  }, []);
}

console.log(slice(data, 0, 3));
console.log(slice(data, 3));
.as-console-wrapper{min-height: 100%!important; top: 0}

user_877130
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