lib: fix modulo sign in webidl convertToInt

The modulo() helper in lib/internal/webidl.js is documented to implement
ECMA-262's "x modulo y" (result has the same sign as y), but it used the
JS % operator directly, whose result takes the sign of x. convertToInt
step 10 relies on modulo(x, 2^bitLength) producing a non-negative
representative in [0, 2^bitLength); step 11 only handles the signed
upper half, so negative results of % were returned unchanged.

As a result, negative inputs to unsigned WebIDL integer types returned
the original negative value instead of the two's-complement wrap (e.g.
convertToInt('v', -3, 8) returned -3 instead of 253), and signed inputs
in the one-wrap band below the lower bound were returned as-is
(convertToInt('v', -200, 8, { signed: true }) returned -200 instead of
56). This diverged from typed-array coercion.

Co-Authored-By: Nikita Skovoroda <chalkerx@gmail.com>
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: DeepView Autofix <276251120+deepview-autofix@users.noreply.github.com>
Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: Nikita Skovoroda <chalkerx@gmail.com>

Author: 3 people

lib/internal/webidl.js

@@ -108,11 +108,14 @@ function pow2(exponent) {
 
 // https://tc39.es/ecma262/#eqn-modulo
 // The notation “x modulo y” computes a value k of the same sign as y.
+// As y is always positive in this file, we can do a simple r < 0 check for that.
 function modulo(x, y) {
   const r = x % y;
   // Convert -0 to +0.
   if (r === 0) {
     return 0;
+  } else if (r < 0) {
+    return r + y;
   }
   return r;
 }
@@ -192,6 +195,14 @@ function convertToInt(name, value, bitLength, options = kEmptyObject) {
   // 9. Set x to IntegerPart(x).
   x = integerPart(x);
 
+  // Following the exact algorithm looses precision on signed modulo in 64-bit
+  // as we operate in doubles. modulo(-1, 2**64) should be 2**64 - 1, but for
+  // signed -1 that gives -1 + 2**64 - 2**64 = 0, instead of -1. We need the
+  // conversion only for values outside of range, and can short-cut otherwise.
+  if (x >= lowerBound && x <= upperBound) {
+    return x === 0 ? 0 : x;
+  }
+
   // 10. Set x to x modulo 2^bitLength.
   x = modulo(x, pow2(bitLength));
 

test/parallel/test-internal-webidl-converttoint.js

@@ -56,3 +56,12 @@ assert.strictEqual(convertToInt('x', 0xFFFF_FFFF, 32), 0xFFFF_FFFF);
 // Out of range, step 11.
 assert.strictEqual(convertToInt('x', 0x8000_0000, 32, { signed: true }), -0x8000_0000);
 assert.strictEqual(convertToInt('x', 0xFFF_FFFF, 32, { signed: true }), 0xFFF_FFFF);
+
+// Negative values must wrap as two's-complement, matching typed-array behavior.
+assert.strictEqual(convertToInt('x', -3, 8), 253);
+assert.strictEqual(convertToInt('x', -3, 8), new Uint8Array([-3])[0]);
+assert.strictEqual(convertToInt('x', -1, 32), 0xFFFF_FFFF);
+assert.strictEqual(convertToInt('x', -1, 32), new Uint32Array([-1])[0]);
+assert.strictEqual(convertToInt('x', -200, 8, { signed: true }), 56);
+assert.strictEqual(convertToInt('x', -200, 8, { signed: true }),
+                   new Int8Array([-200])[0]);