what can you use as keys in an array code example

Example 1: .keys() array

var arr = ['a', , 'c'];
var sparseKeys = Object.keys(arr);
var denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys);  // [0, 1, 2]

Example 2: array of array key value javascript

var categories = [
    {"1":"Category 1"},
    {"2":"Category 2"},
    {"3":"Category 3"},
    {"4":"Category 4"}
];