In the context of JavaScript it means some numeric keys are missing (between 0 and the highest numeric key). This is an issue because JavaScript Arrays are not

In the context of JavaScript it means some numeric keys are missing (between 0 and the highest numeric key). This is an issue because JavaScript Arrays are not like arrays in other languages - they are dictionaries where the keys happen to be numeric. So it is possible for a numeric key not to be defined, which is different from it being defined but having "undefined" as a value.