XS returns the wrong result for expressions that assign to array length

[gibson042]

Environment: XS 13.0.0 32 4

Description
Evaluating array.length = nonNumber does not conform with the ECMAScript specification.

Steps to Reproduce

typeof ([].length = "4")

Actual behavior
"number"

Expected behavior
"string"

Other information
Evaluation of AssignmentExpression : LeftHandSideExpression `=` AssignmentExpression when |LeftHandSideExpression| is not an object or array literal and the right-hand-side |AssignmentExpression| is not a function definition is specified in step 1.e to return the result of evaluating that |AssignmentExpression| in step 1.c.

Basically, even though the intervening PutValue can perform arbitrary processing on that value (as is the case when setting the length of an array ends up in ArraySetLength and the value is coerced to a uint32 number), the assignment expression itself must evaluate to the input of that process rather than its output.

Note also that this affects all assignment operators, e.g. Array(1).length += "e2" should be the string "1e2".

[phoddie]

Thanks for the report.

XS is very deliberate about returning a number from the Array length setter. Here's the relevant code:

moddable/xs/sources/xsArray.c

Lines 996 to 997 in 2420f19

	mxResult->value.number = array->value.array.length;
	mxResult->kind = XS_NUMBER_KIND;

Changing that to return the input argument gives the expected result for the two test cases you provide:

mxResult->value = mxArgv(0)->value;
mxResult->kind = mxArgv(0)->kind;

But, we'll need to take a more careful look.

[phoddie]

Fixed