What is holey array: an array with hole(s)
const array = [1,2, ,3]
Why this is a problem? Should array[2] to be undefined?
Yes and no.. normally it is undefined, but before that, VM has been check Array.prototypeto see Array.prototype["2"]whether it has value assigned or not. If not then VM set it to undefined, otherwise not.
(() => {
const array = [1, 2, , 3];
let sum = 0;
for (let i = 0; i < array.length; i++) {
const value = array[i];
if (typeof value == "number") {
sum += value;
}
}
console.log("Sum:", sum); // 6
})();
Now try to mass up Array.prototype:
(() => {
const array = [1, 2, , 3];
Array.prototype["2"] = -3; // <-- this line
let sum = 0;
for (let i = 0; i < array.length; i++) {
const value = array[i];
if (typeof value == "number") {
sum += value;
}
}
console.log("Sum:", sum); // 3
})();
Is there any other way to create a holey array?
Yes: Array.from(10), once you create a holey array, then all the rest operation .map, .filter, .reducewill also build based on this holey array.
How to prevent that?
Array.from(10, (_el, _i) => 0) // give a defualt value when init the array