<header>
<h1>Flat</h1>
</header>
<body>
<script>
arr = [];
for (let i = 0; i < 5; i++) {
arr[i] = [];
for (let j = 0; j < 5; j++) {
arr[i][j] = (i + 1) * (j + 1);
}
}
const arrCopy = [...arr[0], ...arr[1], ...arr[2], ...arr[3], ...arr[4]];
alert(arrCopy);
</script>
</body>