Properly handle failures

As discussed in web-platform-tests/wpt#2830 (comment). This makes the setter tests pass (by having most of them ignore invalid input instead of throwing). It should also make parsing failures during construction throw correctly; previously they were checking a nonexistant urlRecord.failure property.

This changes the public API by making the URL-record-returning functions return either a URL record or the string "failure", instead of returning either a URL record or throwing.

Author: domenic

README.md

@@ -62,3 +62,5 @@ The URL record type has the following API:
 - [`cannotBeABaseURL`](https://url.spec.whatwg.org/#url-cannot-be-a-base-url-flag) (as a boolean)
 
 These properties should be treated with care, as in general changing them will cause the URL record to be in an inconsistent state until the appropriate invocation of `basicURLParse` is used to fix it up. You can see examples of this in the URL Standard, where there are many step sequences like "4. Set context object’s url’s fragment to the empty string. 5. Basic URL parse _input_ with context object’s url as _url_ and fragment state as _state override_." In between those two steps, a URL record is in an unusable state.
+
+The return value of "failure" in the spec is represented by the string `"failure"`. That is, functions like `parseURL` and `basicURLParse` can return _either_ a URL record _or_ the string `"failure"`.

lib/URL-impl.js

@@ -9,13 +9,13 @@ exports.implementation = class URLImpl {
     let parsedBase = null;
     if (base !== undefined) {
       parsedBase = usm.basicURLParse(base);
-      if (parsedBase.failure) {
+      if (parsedBase === "failure") {
         throw new TypeError("Invalid base URL");
       }
     }
 
     const parsedURL = usm.basicURLParse(url, { baseURL: parsedBase });
-    if (parsedURL.failure) {
+    if (parsedURL === "failure") {
       throw new TypeError("Invalid URL");
     }
 
@@ -45,7 +45,12 @@ exports.implementation = class URLImpl {
   }
 
   set href(v) {
-    this._url = usm.basicURLParse(v);
+    const parsedURL = usm.basicURLParse(v);
+    if (parsedURL === "failure") {
+      throw new TypeError("Invalid URL");
+    }
+
+    this._url = parsedURL;
   }
 
   get origin() {

src/url-state-machine.js

@@ -1129,7 +1129,7 @@ module.exports.basicURLParse = function (input, options) {
 
   const usm = new URLStateMachine(input, options.baseURL, options.encodingOverride, options.url, options.stateOverride);
   if (usm.failure) {
-    throw new TypeError("Invalid URL");
+    return "failure";
   }
 
   return usm.url;