not null but undefined

[Vemus]

Chapter 4: Data Structures: Objects and Arrays; Code Sandbox: 1: The Sum of a Range; Line 2.

[marijnh]

In JavaScript, x == null and x == undefined do the exact same thing.

[Vemus]

If x === null for this problem, it doesn't accept a missing parameter for step and an empty array is returned. It only works if x === undefined.

[fxfactorial]

@marijnh I would say that for this code sample undefined is more expressive of the code's intention. Parameters which are not explicitly passed get a value of undefined, whereas the null would have required caller to explicitly give it as null. Given that the step is the trailing argument and step is usually optional, I would rather the code check for undefined rather than null, as is common in JavaScript coding conventions.