Checking if a key exists in a JavaScript object?

Question

How do I check if a particular key exists in a JavaScript object or array?

If a key doesn\'t exist, and I try to access it, will it return false? Or throw an error?<

Answer 1

In 'array' world we can look on indexes as some kind of keys. What is surprising the in operator (which is good choice for object) also works with arrays. The returned value for non-existed key is undefined

let arr = ["a","b","c"]; // we have indexes: 0,1,2
delete arr[1];           // set 'empty' at index 1
arr.pop();               // remove last item

console.log(0 in arr,  arr[0]);
console.log(1 in arr,  arr[1]);
console.log(2 in arr,  arr[2]);