Add strip_prefix attr to js_library (#2519)

internal/common/module_mappings.bzl

@@ -74,6 +74,8 @@ def _get_module_mappings(target, ctx):
         # the runfiles directory. This requires the workspace_name to be prefixed on
         # each module root.
         mr = "/".join([p for p in [workspace_name, target.label.package] if p])
+        if hasattr(ctx.rule.attr, "strip_prefix") and ctx.rule.attr.strip_prefix:
+            mr += "/" + ctx.rule.attr.strip_prefix
         if hasattr(ctx.rule.attr, "module_root") and ctx.rule.attr.module_root and ctx.rule.attr.module_root != ".":
             if ctx.rule.attr.module_root.endswith(".ts"):
                 # Validate that sources are underneath the module root.

internal/js_library/js_library.bzl

@@ -98,6 +98,10 @@ _ATTRS = {
     ),
     "package_name": attr.string(),
     "srcs": attr.label_list(allow_files = True),
+    "strip_prefix": attr.string(
+        doc = "Path components to strip from the start of the package import path",
+        default = "",
+    ),
 }
 
 AmdNamesInfo = provider(
@@ -223,7 +227,27 @@ def _impl(ctx):
     ]
 
     if ctx.attr.package_name:
-        path = "/".join([p for p in [ctx.bin_dir.path, ctx.label.workspace_root, ctx.label.package] if p])
+        path = "/".join([
+            p
+            for p in [ctx.bin_dir.path, ctx.label.workspace_root, ctx.label.package]
+            if p
+        ])
+
+        # Strip a prefix from the package require path
+        if ctx.attr.strip_prefix:
+            path += "/" + ctx.attr.strip_prefix
+
+            # Check that strip_prefix contains at least one src path
+            check_prefix = "/".join([p for p in [ctx.label.package, ctx.attr.strip_prefix] if p])
+            prefix_contains_src = False
+            for file in all_files:
+                if file.short_path.startswith(check_prefix):
+                    prefix_contains_src = True
+                    break
+
+            if not prefix_contains_src:
+                fail("js_library %s strip_prefix path does not contain any of the provided sources" % ctx.label)
+
         providers.append(LinkablePackageInfo(
             package_name = ctx.attr.package_name,
             path = path,
@@ -339,6 +363,7 @@ def js_library(
         srcs: the list of files that comprise the package
         package_name: the name it will be imported by. Should match the "name" field in the package.json file.
         deps: other targets that provide JavaScript code
+        strip_prefix: path components to strip from the start of the package import path
         **kwargs: used for undocumented legacy features
     """
 

internal/js_library/test/strip_prefix/BUILD.bazel

@@ -0,0 +1,17 @@
+load("//:index.bzl", "js_library", "nodejs_test")
+
+js_library(
+    name = "stripped_library",
+    package_name = "stripped_library",
+    srcs = ["library/file.js"],
+    strip_prefix = "library",
+)
+
+nodejs_test(
+    name = "test",
+    data = [
+        "test.js",
+        ":stripped_library",
+    ],
+    entry_point = "test.js",
+)

internal/js_library/test/strip_prefix/test.js

@@ -0,0 +1,9 @@
+// Try to load on unstripped path, this should fail
+try {
+    require("stripped_library/library/file");
+} catch (exc) {
+    if (exc.code !== 'MODULE_NOT_FOUND') throw exc;
+}
+
+// Load with the stripped path, this should succeed
+require("stripped_library/file");