API: Allow serving localstripe API from a custom URL

It can be desirable to use localstripe from a user-defined URL with a
custom path, e.g.:

    https://domain/subpath/js.stripe.com/v3/
    https://domain/subpath/v1/payment_intents/
    https://domain/subpath/v1/customers/cus_S5TdpWqIoiuKlq/sources

instead of:

    http://domain:8420/js.stripe.com/v3/
    http://domain:8420/v1/payment_intents/
    http://domain:8420/v1/customers/cus_S5TdpWqIoiuKlq/sources

This can be achieved by using a reverse-proxy for example, but the
JavaScript part of localstripe didn't support it.

This commit fixes that by dynamically computing the base API URL from
which the JavaScript code is loaded.

localstripe/localstripe-v3.js

@@ -15,11 +15,15 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-// First, get the domain from which this script is pulled:
-const LOCALSTRIPE_SOURCE = (function () {
+// First, get the URL base from which this script is pulled:
+const LOCALSTRIPE_BASE_API = (function () {
   const scripts = document.getElementsByTagName('script');
   const src = scripts[scripts.length - 1].src;
-  return src.match(/https?:\/\/[^\/]*/)[0];
+  if (src.match(/\/js\.stripe\.com\/v3\/$/)) {
+    return src.replace(/\/js\.stripe\.com\/v3\/$/, '');
+  } else {
+    return src.match(/https?:\/\/[^\/]*/)[0];
+  }
 })();
 
 // Check and warn if the real Stripe is already used in webpage
@@ -210,7 +214,7 @@ Stripe = (apiKey) => {
       body.push('payment_user_agent=localstripe');
       body = body.join('&');
       try {
-        const url = `${LOCALSTRIPE_SOURCE}/v1/tokens`;
+        const url = `${LOCALSTRIPE_BASE_API}/v1/tokens`;
         const response = await fetch(url, {
           method: 'POST',
           headers: {'Content-Type': 'application/x-www-form-urlencoded'},
@@ -233,7 +237,7 @@ Stripe = (apiKey) => {
     createSource: async (source) => {
       console.log('localstripe: Stripe().createSource()');
       try {
-        const url = `${LOCALSTRIPE_SOURCE}/v1/sources`;
+        const url = `${LOCALSTRIPE_BASE_API}/v1/sources`;
         const response = await fetch(url, {
           method: 'POST',
           body: JSON.stringify({
@@ -262,7 +266,7 @@ Stripe = (apiKey) => {
       console.log('localstripe: Stripe().confirmCardSetup()');
       try {
         const seti = clientSecret.match(/^(seti_\w+)_secret_/)[1];
-        const url = `${LOCALSTRIPE_SOURCE}/v1/setup_intents/${seti}/confirm`;
+        const url = `${LOCALSTRIPE_BASE_API}/v1/setup_intents/${seti}/confirm`;
         if (data.payment_method.card instanceof Element) {
           const element = data.payment_method.card;
           data.payment_method.card = element.value.card;
@@ -295,8 +299,8 @@ Stripe = (apiKey) => {
           const url =
             (await openModal('3D Secure\nDo you want to confirm or cancel?',
                              'Complete authentication', 'Fail authentication'))
-            ? `${LOCALSTRIPE_SOURCE}/v1/setup_intents/${seti}/confirm`
-            : `${LOCALSTRIPE_SOURCE}/v1/setup_intents/${seti}/cancel`;
+            ? `${LOCALSTRIPE_BASE_API}/v1/setup_intents/${seti}/confirm`
+            : `${LOCALSTRIPE_BASE_API}/v1/setup_intents/${seti}/cancel`;
           response = await fetch(url, {
             method: 'POST',
             body: JSON.stringify({
@@ -341,7 +345,7 @@ Stripe = (apiKey) => {
           '3D Secure\nDo you want to confirm or cancel?',
           'Complete authentication', 'Fail authentication');
         const pi = clientSecret.match(/^(pi_\w+)_secret_/)[1];
-        const url = `${LOCALSTRIPE_SOURCE}/v1/payment_intents/${pi}` +
+        const url = `${LOCALSTRIPE_BASE_API}/v1/payment_intents/${pi}` +
                     `/_authenticate?success=${success}`;
         const response = await fetch(url, {
           method: 'POST',
@@ -373,7 +377,7 @@ Stripe = (apiKey) => {
       console.log('localstripe: Stripe().confirmSepaDebitSetup()');
       try {
         const seti = clientSecret.match(/^(seti_\w+)_secret_/)[1];
-        const url = `${LOCALSTRIPE_SOURCE}/v1/setup_intents/${seti}/confirm`;
+        const url = `${LOCALSTRIPE_BASE_API}/v1/setup_intents/${seti}/confirm`;
         let response = await fetch(url, {
           method: 'POST',
           body: JSON.stringify({
@@ -407,4 +411,4 @@ Stripe = (apiKey) => {
 
 console.log('localstripe: The Stripe object was just replaced in the page. ' +
             'Stripe elements created from now on will be fake ones, ' +
-            `communicating with the mock server at ${LOCALSTRIPE_SOURCE}.`);
+            `communicating with the mock server at ${LOCALSTRIPE_BASE_API}.`);