How to check if array element exists or not in javascript?

Use typeof arrayName[index] === 'undefined'

i.e.

if(typeof arrayName[index] === 'undefined') {
    // does not exist
}
else {
    // does exist
}

var myArray = ["Banana", "Orange", "Apple", "Mango"];

if (myArray.indexOf(searchTerm) === -1) {
  console.log("element doesn't exist");
}
else {
  console.log("element found");
}

Someone please correct me if i'm wrong, but AFAIK the following is true:

1. Arrays are really just Objects under the hood of JS
2. Thus, they have the prototype method hasOwnProperty "inherited" from Object
3. in my testing, hasOwnProperty can check if anything exists at an array index.

So, as long as the above is true, you can simply:

const arrayHasIndex = (array, index) => Array.isArray(array) && array.hasOwnProperty(index);

usage:

arrayHasIndex([1,2,3,4],4); outputs: false

arrayHasIndex([1,2,3,4],2); outputs: true