[Solved] TypeError: array[i] is undefined

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You are not allowed to put a PHP array directly to JavaScript.
Please try to json_encode it:

onclick='mostrarModal(".$idboton.",".json_encode($arraynombres).");'

Regarding your JavaScript I would suggest to directly use eventos instead of copying the elements to array:

for(var i =0; i < eventos.length;i++) {
    acumuladordenombres = acumuladordenombres +' '+ eventos[i][0];
}

solved TypeError: array[i] is undefined