usage of spread syntax code with map function

document.addEventListener('DOMContentLoaded', () => {

  var temp = [];
  var arrayOfStrings = [];

  console.log(document.querySelectorAll('#wrapper button')) //choose button only in the wrapper div and not the others like CLR
  //listen to only button events(other than the CLR)
  document.querySelectorAll('#wrapper button').forEach(button => button.addEventListener('click', Calculat));
  //listen to CLR button events
  document.getElementById('clear1').addEventListener('click', () => {
    temp = [];
    arrayOfStrings = [];
    document.querySelector('.my_flex').textContent = ('');
  });

  function Calculat(e) {
    z = e.target.innerText;
    //   console.log(z);
    document.querySelector('.my_flex').append(z); //append to the flex object
    if (z == '=') {
      document.querySelector('.my_flex').append(eval(arrayOfStrings));
      return false;
    }
    temp.push(z);
    arrayOfStrings = temp.join("");
  }

});

div {
  width: 7cm;
}

button {
  border: 0;
  line-height: 3.5;
  padding: 0 20px;
  font-size: 1rem;
  text-align: center;
  /*align the text f the button to the center*/
  color: #fff;
  text-shadow: 1px 1px 1px #000;
  border-radius: 10px;
  background-color: rgba(220, 0, 0, 1);
  background-image: linear-gradient(to top left, rgba(0, 0, 0, .2), rgba(0, 0, 0, .2) 30%, rgba(0, 0, 0, 0));
  box-shadow: inset 2px 2px 3px rgba(255, 255, 255, .6), inset -2px -2px 3px rgba(0, 0, 0, .6);
}

button:hover {
  background-color: green;
}

button {
  border: 0;
  line-height: 3.5;
  padding: 0 20px;
  font-size: 1rem;
  text-align: center;
  color: #fff;
  text-shadow: 1px 1px 1px #000;
  border-radius: 10px;
  background-color: rgba(220, 0, 0, 1);
  background-image: linear-gradient(to top left, rgba(0, 0, 0, .2), rgba(0, 0, 0, .2) 30%, rgba(0, 0, 0, 0));
  box-shadow: inset 2px 2px 3px rgba(255, 255, 255, .6), inset -2px -2px 3px rgba(0, 0, 0, .6);
}

button:hover {
  background-color: green;
}

.my_flex {
  display: flex;
  background-color: tomato;
  margin: 8px;
  padding: 20px;
  font-size: 30px;
}

<a>Calculations(Simple Mathematics only, example: 1+7,2*8 etc):--</a>
<div id="wrapper">
  <button>1</button>
  <button>2</button>
  <button>3</button>
  <button>4</button>
  <br>
  <button>5</button>
  <button>6</button>
  <button>7</button>
  <button>8</button>
  <button>9</button>
  <button>+</button>
  <button>-</button>
  <button>*</button>
  <button>0</button>
  <button>(</button>
  <button>)</button>
  <button>/</button>
  <button>=</button>
  <button>.</button>
</div>
<div><button id="clear1">CLEAR</button></div>
<div class="my_flex">
  <!--just to get nice output on flex canvas -->
</div>