Build config: allow hardcoding BACKEND_URL into the build

app/soapbox/api.js

@@ -4,6 +4,8 @@ import axios from 'axios';
 import LinkHeader from 'http-link-header';
 import { getAccessToken, getAppToken, parseBaseURL } from 'soapbox/utils/auth';
 import { createSelector } from 'reselect';
+import { BACKEND_URL } from 'soapbox/build_config';
+import { isURL } from 'soapbox/utils/auth';
 
 export const getLinks = response => {
   const value = response.headers.link;
@@ -33,7 +35,8 @@ const getAuthBaseURL = createSelector([
 
 export const baseClient = (accessToken, baseURL = '') => {
   return axios.create({
-    baseURL,
+    // When BACKEND_URL is set, always use it.
+    baseURL: isURL(BACKEND_URL) ? BACKEND_URL : baseURL,
     headers: Object.assign(accessToken ? {
       'Authorization': `Bearer ${accessToken}`,
     } : {}),

app/soapbox/build_config.js

@@ -0,0 +1,23 @@
+// @preval
+/**
+ * Build config: configuration set at build time.
+ * @module soapbox/build_config
+ */
+
+const { BACKEND_URL } = process.env;
+
+const sanitizeURL = url => {
+  try {
+    return new URL(url).toString();
+  } catch {
+    return '';
+  }
+};
+
+// JSON.parse/stringify is to emulate what @preval is doing and avoid any
+// inconsistent behavior in dev mode
+const sanitize = obj => JSON.parse(JSON.stringify(obj));
+
+module.exports = sanitize({
+  BACKEND_URL: sanitizeURL(BACKEND_URL),
+});