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constructor and all of the static methods on the Object constructor (a.k.a.
the Object global):
Object.assign Object.create
Object.defineProperties Object.defineProperty
Object.entries Object.freeze
Object.fromEntries Object.getOwnPropertyDescriptor
Object.getOwnPropertyDescriptors Object.getOwnPropertyNames
Object.getOwnPropertySymbols Object.getPrototypeOf
Object.groupBy Object.hasOwn
Object.is Object.isExtensible
Object.isFrozen Object.isSealed
Object.keys Object.length
Object.name Object.preventExtensions
Object.prototype Object.seal
Object.setPrototypeOf Object.values
----[ 5.1 Dynamic function calling
At first these all seem relatively boring, but an unexpected hero here is
Object.groupBy() [2]:
Object.groupBy(items, callbackFn)
"The Object.groupBy() static method groups the elements of a given
iterable according to the string values returned by a provided callback
function. The returned object has separate properties for each group,
containing arrays with the elements in the group."
This relatively new addition to the JavaScript language does a bunch of
things we'll end up needing. Most importantly it will call a function for
us, solving the second problem from before.
For example, if we somehow manage to obtain a useful Function instance, we
can call it like this to run the JavaScript code:
x = Function("alert('hello world')"); // assuming we have this string
Object.groupBy([1],x) // will call x for us
This only leaves us with the first problem: constructing an arbitrary
string. It turns out that this is the hard part.
----[ 5.2 First steps in constructing a string
The method String.fromCharCode immediately comes to mind. It returns a
string consisting of one or more UTF-16 code units passed as arguments:
> String.fromCharCode(0x41, 0x42, 0x43)
'ABC'
This would be perfect for us, as integers are no problem for WASM. Though,
the problem is of course that fromCharCode is part of the String global,
not Object. Luckily, there is a way to obtain it. We'll need to perform the
following operations, sketched out in JavaScript for readability:
// Obtain any string
str = "foobar";
// Get the String constructor (i.e. the String global)
string_constructor = str.constructor;
// Get String.fromCharCode
fromCharCode = string_constructor.fromCharCode;
Obtaining an initial string is a bit tricky, but doable. For example, to
get the literal string "length", we can do the following:
empty_object = importObject.constructor.prototype; // {}, importable
empty_array = Object.values(empty_object); // []
Object.getOwnPropertyNames(empty_array); // [ 'length' ]
The result is still wrapped in an array, but we can use Object.groupBy to
help with that. A key insight here is that we can craft arbitrary callback
functions because it is perfectly legal to pass a WASM function using
ref.func instead of a JavaScript function reference (an externref). So, we
could craft a "save-the-nth-element" function by looking at the second
argument passed to the callback, the index. In pseudo-JavaScript:
g_n = null;
save_first_elem = (x, i) => (if(i == 0) {g_n = x});
arr = [ 'length' ];
Object.groupBy(arr, save_first_elem);
// g_n == 'length'
Generalized, we can define $array_get_nth_element as follows in WASM:
;; Callback to use with Object.groupBy() to extract element $n to $g_n.
(func $save_nth_element (param $val externref) (param $n i32)
(local.get $n)
(global.get $g_n)
i32.eq
(if
(then
(local.get $val)
(global.set $g_nth_element)
)
)
)
;; Given $arr and $n, return $arr[n]