fix(webidl): correct signed integer bounds in ConvertToInt

lib/web/webidl/index.js

@@ -188,10 +188,10 @@ webidl.util.ConvertToInt = function (V, bitLength, signedness, flags) {
   } else {
     // 3. Otherwise:
 
-    // 1. Let lowerBound be -2^bitLength − 1.
-    lowerBound = Math.pow(-2, bitLength) - 1
+    // 1. Let lowerBound be -2^(bitLength − 1).
+    lowerBound = -Math.pow(2, bitLength - 1)
 
-    // 2. Let upperBound be 2^bitLength − 1 − 1.
+    // 2. Let upperBound be 2^(bitLength − 1) − 1.
     upperBound = Math.pow(2, bitLength - 1) - 1
   }
 
@@ -270,9 +270,9 @@ webidl.util.ConvertToInt = function (V, bitLength, signedness, flags) {
   // 10. Set x to x modulo 2^bitLength.
   x = x % Math.pow(2, bitLength)
 
-  // 11. If signedness is "signed" and x ≥ 2^bitLength − 1,
+  // 11. If signedness is "signed" and x ≥ 2^(bitLength − 1),
   //    then return x − 2^bitLength.
-  if (signedness === 'signed' && x >= Math.pow(2, bitLength) - 1) {
+  if (signedness === 'signed' && x >= Math.pow(2, bitLength - 1)) {
     return x - Math.pow(2, bitLength)
   }
 

test/webidl/util.js

@@ -125,4 +125,23 @@ test('webidl.util.ConvertToInt(V)', () => {
   }
 
   assert.equal(ConvertToInt(111, 2, 'signed'), -1)
+
+  // https://webidl.spec.whatwg.org/#abstract-opdef-converttoint
+  // For N-bit signed integers, lowerBound is -2^(N-1) and the modulo wrap
+  // threshold (step 11) is 2^(N-1).
+  assert.equal(ConvertToInt(128, 8, 'signed'), -128, '8-bit signed wrap at 128')
+  assert.equal(ConvertToInt(200, 8, 'signed'), -56, '8-bit signed wrap at 200')
+  assert.equal(
+    ConvertToInt(-128, 8, 'signed', webidl.attributes.EnforceRange),
+    -128,
+    '8-bit signed EnforceRange lower bound'
+  )
+  assert.throws(() => {
+    ConvertToInt(-129, 8, 'signed', webidl.attributes.EnforceRange)
+  }, TypeError, '8-bit signed EnforceRange below lower bound throws')
+  assert.equal(
+    ConvertToInt(-(2 ** 31), 32, 'signed', webidl.attributes.EnforceRange),
+    -(2 ** 31),
+    '32-bit signed EnforceRange lower bound'
+  )
 })