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(func $array_get_nth_element (param $arr externref) (param $n i32)
(result externref)
(local.get $n)
(global.set $g_n)
(call $groupBy_i (local.get $arr) (ref.func $save_nth_element))
drop
(global.get $g_nth_element)
)
We use the name $groupBy_i to indicate the case where we pass an internal
(WASM) function, whereas we will use $groupBy_e for calling external
function references. Luckily, it is perfectly fine to import the same name
twice with different types!
(import "constructor" "groupBy" (func $groupBy_i
(param externref) (param funcref) (result externref)))
(import "constructor" "groupBy" (func $groupBy_e
(param externref) (param externref) (result externref)))
So, using this mechanism, the string "length" can be grabbed as follows:
;; Obtain the string 'length'
;; Luckily it is the only enumerable object of an empty array, so idx: 0
(call $array_get_nth_element
(call $getOwnPropertyNames
(call $values (global.get $prototype))
)
(i32.const 0)
)
(global.set $s_length)
----[ 5.3 Extracting named properties
Next, we need a way to access named properties (e.g. .constructor and
.fromCharCode) of an object.
For this, we can use Object.getOwnPropertyDescriptors() to get an
"expanded" version of the object, with all of its properties as
descriptors. Running Object.values() on that then gives these descriptors
as a list:
> Object.values(Object.getOwnPropertyDescriptors(string_constructor))
[
{
value: 'String',
writable: false,
enumerable: false,
configurable: true
},
{
value: [Function: fromCharCode],
writable: true,
enumerable: false,
configurable: true
},
{
value: [Function: fromCodePoint],
writable: true,
enumerable: false,
configurable: true
},
...
]
If we know the order of this list, it is then purely a matter of using our
$array_get_nth_element function (e.g., with index 1), giving us just the
descriptor we want:
{
value: [Function: fromCharCode],
writable: true,
enumerable: false,
configurable: true
}
We then run Object.values() on this and use $array_get_nth_element again
(with index 0) to get the property value we desire (here, the fromCharCode
function itself). In reality, the order of this list differs per JavaScript
engine but we'll solve that problem later.
----[ 5.4 Obtaining individual characters
Now that we have a reference to String.fromCharCode, we can call it with
Object.groupBy():
> Object.groupBy([0x42], String.fromCharCode)
{ 'B\x00': [ 66 ] }
The fact that the return value is given as a key of an object is not a
problem, we can use Object.keys() and $array_get_nth_element for that. The
\x00 (due to Object.groupBy() passing the element's index as the second
argument of String.fromCharCode) can also be removed by taking the first
"element" (i.e., character) of the string using $array_get_nth_element,
leaving us with just the string 'B'.
To wrap the input value (0x42) in an array for use with Object.groupBy(),
we perform some more trickery: another call to Object.groupBy() with a WASM
callback allows us to produce the object { "66": ["length"] }, which we can