Warning when manifest key prefix starts with "/"

lib/WebpackConfig.js

@@ -11,6 +11,7 @@
 
 const path = require('path');
 const fs = require('fs');
+const logger = require('./logger');
 
 /**
  * @param {RuntimeConfig} runtimeConfig
@@ -107,6 +108,18 @@ class WebpackConfig {
     }
 
     setManifestKeyPrefix(manifestKeyPrefix) {
+        /*
+         * Normally, we make sure that the manifest keys don't start
+         * with an opening "/" ever... for consistency. If you need
+         * to manually specify the manifest key (e.g. because you're
+         * publicPath is absolute), it's easy to accidentally add
+         * an opening slash (thereby changing your key prefix) without
+         * intending to. Hence, the warning.
+         */
+        if (manifestKeyPrefix.indexOf('/') === 0) {
+            logger.warning(`The value passed to setManifestKeyPrefix "${manifestKeyPrefix}" starts with "/". This is allowed, but since the key prefix does not normally start with a "/", you may have just changed the prefix accidentally.`);
+        }
+
         // guarantee a single trailing slash, except for blank strings
         if (manifestKeyPrefix !== '') {
             manifestKeyPrefix = manifestKeyPrefix.replace(/\/$/, '');

test/WebpackConfig.js

@@ -15,6 +15,7 @@ const RuntimeConfig = require('../lib/config/RuntimeConfig');
 const path = require('path');
 const fs = require('fs');
 const webpack = require('webpack');
+const logger = require('../lib/logger');
 
 function createConfig() {
     const runtimeConfig = new RuntimeConfig();
@@ -148,10 +149,10 @@ describe('WebpackConfig object', () => {
 
         it('You can set it!', () => {
             const config = createConfig();
-            config.setManifestKeyPrefix('/foo');
+            config.setManifestKeyPrefix('foo');
 
             // trailing slash added
-            expect(config.manifestKeyPrefix).to.equal('/foo/');
+            expect(config.manifestKeyPrefix).to.equal('foo/');
         });
 
         it('You can set it blank', () => {
@@ -161,6 +162,15 @@ describe('WebpackConfig object', () => {
             // trailing slash not added
             expect(config.manifestKeyPrefix).to.equal('');
         });
+
+        it('You can use an opening slash, but get a warning', () => {
+            const config = createConfig();
+
+            logger.reset();
+            logger.quiet();
+            config.setManifestKeyPrefix('/foo/');
+            expect(logger.getMessages().warning).to.have.lengthOf(1);
+        });
     });
 
     describe('addEntry', () => {