De un array numérico, encontrar el número que aparezca un número de veces impar (solo habrá un número que aparezca un número de veces impar)

findOdInt.js

/*
Given an array of integers, find the one that appears an odd number of times.

There will always be only one integer that appears an odd number of times.

Examples
[7] should return 7, because it occurs 1 time (which is odd).
[0] should return 0, because it occurs 1 time (which is odd).
[1,1,2] should return 2, because it occurs 1 time (which is odd).
[0,1,0,1,0] should return 0, because it occurs 3 times (which is odd).
[1,2,2,3,3,3,4,3,3,3,2,2,1] should return 4, because it appears 1 time (which is odd).
*/
const mostrarConsole = false;

function findOdd(A) {
    //happy coding!

    // Si solo hay un elemento, ese es el buscado.
    if (A.length == 1 ) return A[0];

    let a1 = [];
    let v = [2];

    // Agrupar los elementos con el mismo valor.
    for (let i = 0; i < A.length; i++) {
      let valor = A[i];
      let esta = false;
      for (let j = 0; j < a1.length; j++) {
        v = a1[j];
        if (valor == v[0]) {
          esta = true;
          v[1] += 1;
          a1[j] = v;
          break;
        }
      }
      if (esta == false) {
        // Crear una nueva variable para no usar la misma referencia.
        let v1 = [2];
        v1[0] = valor; //A[i];
        v1[1] = 1;
        a1.push(v1);
      }
    }
    for (let i = 0; i < a1.length; i++) {
      v = a1[i];
      if (mostrarConsole)
        console.log(i + " = valor: " + v[0] + " veces: " + v[1]);
      // Si las veces es impar, devolver este número.
      if (v[1] % 2 != 0) return v[0];
    }

    return 0;
}


function doTest(arr, resOK) {
    console.log("[" + arr.toString() + "] = " + resOK);

    let res = findOdd(arr);
    if (res == resOK) {
      if (mostrarConsole)
          console.log("\tEl resultado es OK.");
    }
    else {
        console.log("\tEl resultado calculado es " + res + " debería ser " + resOK);
    }
}

doTest([7], 7);
doTest([0], 0);
doTest([1,1,2], 2);
doTest([0,1,0,1,0], 0);
doTest([1,2,2,3,3,3,4,3,3,3,2,2,1], 4);

doTest([20,1,-1,2,-2,3,3,5,5,1,2,4,20,4,-1,-2,5], 5);
doTest([1,1,2,-2,5,2,4,4,-1,-2,5], -1);
doTest([20,1,1,2,2,3,3,5,5,4,20,4,5], 5);
doTest([10], 10);
doTest([1,1,1,1,1,1,10,1,1,1,1], 10);
doTest([5,4,3,2,1,5,4,3,2,10,10], 1);