Normative: avoid quadratic behavior in fromHexInto (#44)

playground/polyfill-core.mjs

@@ -257,15 +257,12 @@ export function hexToUint8Array(string, into) {
   if (typeof string !== 'string') {
     throw new TypeError('expected string to be a string');
   }
-  if (string.length % 2 !== 0) {
-    throw new SyntaxError('string should be an even number of characters');
-  }
-  if (/[^0-9a-fA-F]/.test(string)) {
-    throw new SyntaxError('string should only contain hex characters');
-  }
   if (into && 'detached' in into.buffer && into.buffer.detached) {
     throw new TypeError('fromHexInto called on array backed by detached buffer');
   }
+  if (string.length % 2 !== 0) {
+    throw new SyntaxError('string should be an even number of characters');
+  }
 
   let maxLength = into ? into.length : 2 ** 53 - 1;
 
@@ -275,7 +272,11 @@ export function hexToUint8Array(string, into) {
   let index = 0;
   if (maxLength > 0) {
     while (index < string.length) {
-      bytes.push(parseInt(string.slice(index, index + 2), 16));
+      let hexits = string.slice(index, index + 2);
+      if (/[^0-9a-fA-F]/.test(hexits)) {
+        throw new SyntaxError('string should only contain hex characters');
+      }
+      bytes.push(parseInt(hexits, 16));
       index += 2;
       if (bytes.length === maxLength) {
         break;

spec.html

@@ -330,11 +330,11 @@ <h1>
       1. If _maxLength_ is not present, let _maxLength_ be 2<sup>53</sup> - 1.
       1. Let _length_ be the length of _string_.
       1. If _length_ modulo 2 is not 0, throw a *SyntaxError* exception.
-      1. If _string_ contains any code units which are not in *"0123456789abcdefABCDEF"*, throw a *SyntaxError* exception.
       1. Let _bytes_ be « ».
       1. Let _index_ be 0.
       1. Repeat, while _index_ &lt; _length_,
         1. Let _hexits_ be the substring of _string_ from _index_ to _index_ + 2.
+        1. If _hexits_ contains any code units which are not in *"0123456789abcdefABCDEF"*, throw a *SyntaxError* exception.
         1. Set _index_ to _index_ + 2.
         1. Let _byte_ be the integer value represented by _hexits_ in base-16 notation, using the letters A-F and a-f for digits with values 10 through 15.
         1. Append _byte_ to _bytes_.