Array.isArray(array)

if (array) {
  // empty arrays evaluate as false
}
​
if (array && array.length > 0) {
  // you may still want to capture the array
}

const clone = array.slice(0);

  const array = ['x', 'y', 'z'];
  for (const key in array) {
    console.log(key);
  }

0
1
2

  const array = ['x', 'y', 'z'];
  for (const value of array) {
    console.log(value);
  }

x
y
z

array1.push(...array2);

const array3 = array1.concat(array2);