Add --auto-base-path flag to build (#190)

packages/cli/CHANGELOG.md

@@ -4,6 +4,7 @@
 ## Unreleased
 * `build`:
   * Disable Babel `minify-constant-folding` plugin when minifying. This plugin has a bug that breaks when a constant is exported from a module (https://github.com/babel/minify/issues/820).
+  * Added `--auto-base-path` flag. Sets the entrypoint `<base>` tag for all builds to match the name of that build. Unlike other flags, does not necessarily trigger a single one-off build.
 <!-- Add new, unreleased items here. -->
 
 ## v1.7.0-pre.13 [04-19-2018]

packages/cli/src/commands/build.ts

@@ -31,6 +31,10 @@ import {Command, CommandOptions} from './command';
 
 const logger = logging.getLogger('cli.command.build');
 
+// This flag is special because it doesn't necessarily trigger a one-off single
+// build.
+const autoBasePath = 'auto-base-path';
+
 export class BuildCommand implements Command {
   name = 'build';
   aliases = [];
@@ -123,6 +127,12 @@ export class BuildCommand implements Command {
       type: String,
       description: 'Updates the <base> tag if found in the entrypoint document.'
     },
+    {
+      name: autoBasePath,
+      type: Boolean,
+      description: 'For all builds, set the entrypoint <base> tag to match ' +
+          'the build name. Does not necessarily trigger a one-off build.',
+    },
   ];
 
   /**
@@ -133,6 +143,10 @@ export class BuildCommand implements Command {
       ProjectBuildOptions {
     const buildOptions: ProjectBuildOptions = {};
     const validBuildOptions = new Set(this.args.map(({name}) => name));
+
+    // This flag is special. It affects the top-level config, not an individual build.
+    validBuildOptions.delete(autoBasePath);
+
     for (const buildOption of Object.keys(options)) {
       if (validBuildOptions.has(buildOption)) {
         const [prefix, ...rest] = buildOption.split('-');
@@ -150,6 +164,9 @@ export class BuildCommand implements Command {
         }
       }
     }
+    if (options[autoBasePath]) {
+      buildOptions.basePath = true;
+    }
     return applyBuildPreset(buildOptions);
   }
 
@@ -181,18 +198,24 @@ export class BuildCommand implements Command {
 
     // If any the build command flags were passed as CLI arguments, generate
     // a single build based on those flags alone.
-    const hasCliArgumentsPassed =
-        this.args.some((arg) => typeof options[arg.name] !== 'undefined');
-    if (hasCliArgumentsPassed) {
+    const hasOneOffBuildArgumentPassed = this.args.some((arg) => {
+      return typeof options[arg.name] !== 'undefined' &&
+          arg.name !== autoBasePath;
+    });
+    if (hasOneOffBuildArgumentPassed) {
       await build(this.commandOptionsToBuildOptions(options), polymerProject);
       return;
     }
 
     // If no build flags were passed but 1+ polymer.json build configuration(s)
     // exist, generate a build for each configuration found.
     if (config.builds) {
-      const promises = config.builds.map(
-          (buildOptions) => build(buildOptions, polymerProject));
+      const promises = config.builds.map((buildOptions) => {
+        if (options[autoBasePath]) {
+          buildOptions.basePath = true;
+        }
+        return build(buildOptions, polymerProject);
+      });
       promises.push(mzfs.writeFile(
           path.join(mainBuildDirectoryName, 'polymer.json'), config.toJSON()));
       await Promise.all(promises);