Array

Array.isArray(array)
This does not work to test if its an 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

Reuse Array:
array1.push(...array2);
New Array:
const array3 = array1.concat(array2);
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On this page
Test Array
Clone an array
Iterate keys in an array (for...in)
Iterate values in an array (for...of)
Appending/merging arrays together