JavaScript How to replace first element of an array with each element of another array

Question

I have two arrays and I need to replace the first element of the first array with each elements of the second array:

let firstArray = [
  [1, 'a', 'hello'],
  [2, 'b', 'world'],
  [3, 'c', 'other'],
  ...
];

let secondArray = [1, 3, 7, ...];

// Result: 
// [
//  [1, 'a', 'hello'],
//  [3, 'b', 'world'],
//  [7, 'c', 'other'],
//  ...
// ]

I tried doing something like this:

firstArray.map(f => {
  secondArray.forEach(s => {
      f.splice(0, 1, s);
  })
})

But this replace the first element only with the last element of second array

Also, post your expected output to make it clear.

palaѕн
– palaѕн

2020-05-12 04:50:36 +00:00
Commented May 12, 2020 at 4:50 — palaѕн
– palaѕн, Commented May 12, 2020 at 4:50

CertainPerformance · Accepted Answer · 2020-05-12 04:51:15Z

1

Use .map to transform an array into another:

const firstArray = [
  [1, 'a', 'hello'],
  [2, 'b', 'world'],
  [3, 'c', 'other'],
];

const secondArray = [1, 3, 7];

const transformed = firstArray.map(([, ...rest], i) => [secondArray[i], ...rest]);
console.log(transformed);

Another option:

const firstArray = [
  [1, 'a', 'hello'],
  [2, 'b', 'world'],
  [3, 'c', 'other'],
];

const secondArray = [1, 3, 7];

const transformed = firstArray.map((item, i) => [secondArray[i]].concat(item.slice(1)));
console.log(transformed);

answered May 12, 2020 at 4:51

CertainPerformance

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Comments

Nina Scholz · Accepted Answer · 2020-05-12 04:57:51Z

0

You could assign the array to a new array and take the new value to a specified index.

const
    firstArray = [[1, 'a', 'hello'], [2, 'b', 'world'], [3, 'c', 'other']],
    secondArray = [1, 3, 7],
    result = firstArray.map((a, i) => Object.assign([], a, { 0: secondArray[i] }));

console.log(result);

answered May 12, 2020 at 4:57

Nina Scholz

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