Here’s what I have so far:

In JavaScript, arrays are not primitives but are instead represented with Array objects, which have the following core characteristics:

• JavaScript arrays are not fixed-length and not restricted to containing a single data type but can have a mix of data types. (When those characteristics are undesirable, use typed arrays instead.)

• JavaScript arrays are zero-indexed: the first element of a JavaScript array is at index 0, and the last element is at the value of the array's {{jsxref("Array.length", "length")}} property minus1. See the access an array item by its index example. Using an invalid or out-range index number returns undefined.

• JavaScript array-copy operations create shallow copies — (just as copy operations with any JavaScript objects create shallow copies): assigning an existing array to a new variable does not create a copy. Instead the new variable contains a_reference_ to the original array. If you change a value in the original array, it will be reflected in the new array. See the copy an array examples.

"In JavaScript, arrays are not primitives but are instead represented with Array objects"

This reminds me of when I was first learning to program, in JS. And it was a while before I realized that arrays and regexes are 'just objects', in my mind because they had a literal version that meant they were a primitive

I think that, if you don't want to explicitly call out arrays-in-arrays and the things that can represent, then this line covers it

JavaScript arrays are not fixed-length and not restricted to containing a single data type but can have a mix of data types.

It could something to the effect that '... and can hold any mix of javascript values, including primitives, objects, functions, and other arrays'. If someone is learning about arrays for the first time, multidimensional sounds complicated. The flexibility should be explicit.

the usual way of doing multidimensional arrays on languages without the distinction between multidimensional and jagged arrays actually is to use stride (aka pitch); you probably saw something about it if you ever did 2d graphics

nested arrays are actually pretty weird as far as representing “data with multiple dimensions” go, or at least I wouldn’t consider, say, an array of pairs an n-by-2 multidimensional array

this is more of a philosophical discussion, though 😇

foo[1] = {"foo": "bar"};  // assigns a completely new value

// alters the existing foo[1] value
foo[1].foo = "bar";
delete foo[1].hoge;