Reduce an array of arrays (Javascript)

I have an array of arrays i.e:
const arrayDaddy = [[x, 1], [x, 1], [y, 2], [y, 2], [y, 2], [z, 3]]

my end goal is to take the above arrayDaddy and mutate it by adding the number in arrayDaddy[i][1] if the two items have the same value arrayDaddy[i][0]. Additionally, I want to throw out nested arrays with repeated arrayDaddy[i][0] values after they have been added. The desired result of this process on arrayDaddy would be as follows:

arrayDaddy = [[x, 2], [y, 6], [z, 3]]

I am wondering if there is a way to use Array.prototype.reduce(); to achieve this. If not, what is a simple way to achieve this goal?

NOTE: not sure if this is important, but all items with the same value in index 0 will share an index 1 value. for example consider [z, 3]. any addition [z, ] items will also have a 3 in index 1.

I did try reduce(); but did not have success. my code looked something like this:

const arrayDaddy = [[x, 1], [x, 1], [y, 2], [y, 2], [y, 2], [z, 3]] ;

arrayDaddy.reduce((arr1, arr2) => {
  if(arr1[0] === arr2[0]){
    arr1[1] += arr2[1]
    // do something here to delete arr2... but I can't use delete on a local variable?
  } else {
    //do nothing, continue to the next arr in arrayDaddy
  }
});

I also considered map, but want to avoid creating a new array. I want to mutate the arrayDaddy

You could use reduce to generate. First you must store the values by the key who is the first element of the subarray. For example, with Map. So you will update the Map in the reduce and when is completed transform it to an Array.

NOTE: The elements of the Map are ordered by the creation of their key.

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const arrayDaddy = [["x", 1], ["x", 1], ["y", 2], ["y", 2], ["y", 2], ["z", 3]];

const res = Array.from(arrayDaddy.reduce((p, c) => {
	const prevVal = p.get(c[0]) || 0;
	p.set(c[0], prevVal + c[1])
	return p;
}, new Map()));

console.log(res);

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No Map

If you really don't want use Map you must need to store a reference of subarrays in an object.

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const arrayDaddy = [["x", 1], ["x", 1], ["y", 2], ["y", 2], ["y", 2], ["z", 3]];

// _tmp is to store the reference and _idx the current loop index
let _tmp = {};
let _idx = 0;
while (_idx < arrayDaddy.length) {
	// Get current element
	const element = arrayDaddy[_idx];

	// If the item is not stored, must store and skip the element
	if (_tmp[element[0]] === undefined) {
		_tmp[element[0]] = element;
		++_idx;
		continue;
	}

	// Update value and drop array element
	_tmp[element[0]][1] += element[1];
	arrayDaddy.splice(_idx, 1);
}

console.log(arrayDaddy);

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No store reference values

If you don't want to store any references to save memory you must iterate each element and check if has a previus one to the index to be able to sum the current value and drop the current element.

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const arrayDaddy = [["x", 1], ["x", 1], ["y", 2], ["y", 2], ["y", 2], ["z", 3]];

// _idx is one because you don't need process the 0 element
let _idx = 1;
while (_idx < arrayDaddy.length) {
	// Get current element
	const element = arrayDaddy[_idx];

	// Check if has previous element
	let prevFind = false;
	for (let pi = _idx - 1; pi >= 0; --pi) {
		// If find, update value, drop array element and set prevFind flag
		if (arrayDaddy[pi][0] === element[0]) {
			arrayDaddy[pi][1] += element[1];
			arrayDaddy.splice(_idx, 1);
			prevFind = true;
			break;
		}
	}

	// No previuos value, go next
	if (!prevFind) {
		++_idx;
	}
}

console.log(arrayDaddy);

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If you have same groups in sequences, you could check the last group and add the value.

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const
[x, y, z] = 'xyz';
array = [[x, 1], [x, 1], [y, 2], [y, 2], [y, 2], [z, 3]],
result = array.reduce((r, [k, v]) => {
if (r.at(-1)?.[0] === k) r.at(-1)[1] += v;
else r.push([k, v]);
return r;
}, []);

console.log(result);

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