Working with bidimensional arrays in JavaScript to detect repeated values

Question

I have been working on an example that detects repetitions by letter-number with the following JavaScript code.

But it's not working. The idea is that it detects that the combination A-1 is repeated; using a control matrix.

This is the items JSON structure:

{
letter: 'A',
number: '1',
},
{
letter: 'B',
number: '2',
},
{
letter: 'A',
number: '1',
},
{
letter: 'C',
number: '2',
},

And this is the code I am trying to:

var isRepeated = false;
var numItems = items.length;
if (numItems > 0) {
    var item; 
    var letter;
    var number;
    var matrix = [[]];
    for (var j = 0; j < numItems; j++) {
        item = items[j];
        letter = item.letter;
        number = item.number;
        if (matrix[letter][number] == undefined) {
            matrix[letter][number] = true;
        } else {
            isRepeated = true;
            break;
        }
    }
}

Answer 1

matrix[letter][number]

You're trying to access elements of an array using string keys but arrays use integer indexes

Here's an alternative if you just want to know if something is repeated

 const props = [{ letter: 'A', number: '1', }, { letter: 'B', number: '2', }, { letter: 'A', number: '1', }, { letter: 'C', number: '2', }, ] const didRepeat = (array) => array.some(((hash) => ({letter, number}) => { const key = `${letter}_${number}`; if (hash.has(key)) return true; return hash.add(key), false; })(new Set())) console.log(didRepeat(props))