How do I check in JavaScript if a value exists at a certain array index?

All arrays in JavaScript contain array.length elements, starting with array[0] up until array[array.length - 1]. By definition, an array element with index i is said to be part of the array if i is between 0 and array.length - 1 inclusive.

That is, JavaScript arrays are linear, starting with zero and going to a maximum, and arrays don't have a mechanism for excluding certain values or ranges from the array. To find out if a value exists at a given position index (where index is 0 or a positive integer), you literally just use

if (index < array.length) {
  // do stuff
}

However, it is possible for some array values to be null, undefined, NaN, Infinity, 0, or a whole host of different values. For example, if you add array values by increasing the array.lengthproperty, any new values will be undefined.

To determine if a given value is something meaningful, or has been defined. That is, not undefined, or null:

if (typeof array[index] !== 'undefined') {

or

if (typeof array[index] !== 'undefined' && array[index] !== null) {

Interestingly, because of JavaScript's comparison rules, my last example can be optimised down to this:

if (array[index] != null) {
  // The == and != operators consider null equal to only null or undefined
}