@@ -33,7 +33,7 @@ def collect_es6_sources(ctx):
         non_rerooted_files += ctx.files.srcs
     for dep in ctx.attr.deps:
         if hasattr(dep, "typescript"):
-            non_rerooted_files += [f.path for f in dep.typescript.transitive_es6_sources]
+            non_rerooted_files += dep.typescript.transitive_es6_sources.to_list() 
 
     rerooted_files = []
     for file in non_rerooted_files:
 
@@ -39,7 +39,7 @@ def _devmode_js_sources_impl(ctx):
 
     ctx.actions.write(ctx.outputs.manifest, "".join([
         expand_path_into_runfiles(ctx, f.path) + "\n"
-        for f in files
+        for f in files.to_list()
     ]))
     return [DefaultInfo(files = files)]
 
 
@@ -42,7 +42,7 @@ def _sources_aspect_impl(target, ctx):
         result = depset(transitive = [result, target.typescript.es5_sources])
     elif hasattr(target, "files"):
         result = depset(
-            [f for f in target.files if f.path.endswith(".js")],
+            [f for f in target.files.to_list() if f.path.endswith(".js")],
             transitive = [result],
         )
     return struct(node_sources = result)
 
@@ -23,8 +23,9 @@ def _mock_typescript_lib(ctx):
     es5_sources = depset()
     transitive_decls = depset()
     for s in ctx.attr.srcs:
-        es5_sources = depset([f for f in s.files if f.path.endswith(".js")], transitive = [es5_sources])
-        transitive_decls = depset([f for f in s.files if f.path.endswith(".d.ts")], transitive = [transitive_decls])
+        files_list = s.files.to_list()
+        es5_sources = depset([f for f in files_list if f.path.endswith(".js")], transitive = [es5_sources])
+        transitive_decls = depset([f for f in files_list if f.path.endswith(".d.ts")], transitive = [transitive_decls])
     return struct(
         runfiles = ctx.runfiles(collect_default = True, collect_data = True),
         typescript = struct(
 
@@ -102,18 +102,17 @@ def _short_path_to_manifest_path(ctx, short_path):
 def _nodejs_binary_impl(ctx):
     node = ctx.file.node
     node_modules = ctx.files.node_modules
-    sources = []
+    
+    # Using a depset will allow us to avoid flattening files and sources
+    # inside this loop. This should reduce the performances hits, 
+    # since we don't need to call .to_list()
+    sources = depset()
+    
     for d in ctx.attr.data:
         if hasattr(d, "node_sources"):
-            # sources += d.node_sources.to_list()
-            sources += [f for f in d.node_sources]
+            sources = depset( transitive = [sources, d.node_sources] )
         if hasattr(d, "files"):
-            # sources += d.files.to_list()
-            sources += [f for f in d.files]
-
-    # print([f for f in d.files])
-    # print(d.files.to_list())
-    # print(sources)
+            sources = depset( transitive = [sources, d.files] )
 
     _write_loader_script(ctx)
 
@@ -152,13 +151,24 @@ def _nodejs_binary_impl(ctx):
         is_executable = True,
     )
 
-    runfiles = depset(sources + [node, ctx.outputs.loader, ctx.file._repository_args] + node_modules + ctx.files._node_runfiles)
+    runfiles = depset(
+        transitive = [sources + [node, ctx.outputs.loader, ctx.file._repository_args] + node_modules + ctx.files._node_runfiles]
+    )
 
     return [DefaultInfo(
         executable = ctx.outputs.script,
         runfiles = ctx.runfiles(
             transitive_files = runfiles,
-            files = [node, ctx.outputs.loader] + ctx.files._source_map_support_files + node_modules + sources,
+            files = [
+                node, 
+                ctx.outputs.loader
+            ] + ctx.files._source_map_support_files + node_modules + 
+            
+            # We need this call to the list of Files. 
+            # Calling the .to_list() method may have some perfs hits,
+            # so we should be running this method only once per rule.
+            # see: https://docs.bazel.build/versions/master/skylark/depsets.html#performance
+            sources.to_list(),
             collect_data = True,
         ),
     )]
 
@@ -97,7 +97,8 @@ def _npm_package(ctx):
 
         deps_sources = depset(transitive = transitive)
 
-    package_dir = create_package(ctx, [f for f in deps_sources], ctx.files.packages)
+    # Note: to_list() should be called once per rule!
+    package_dir = create_package(ctx, deps_sources.to_list(), ctx.files.packages)
 
     return [DefaultInfo(
         files = depset([package_dir]),
 
@@ -151,6 +151,9 @@ def run_rollup(ctx, sources, config, output):
     return map_output
 
 def _filter_js_inputs(all_inputs):
+
+    # Note: make sure that "all_inputs" is not a depset.
+    # Iterating over a depset is deprecated!
     return [
         f
         for f in all_inputs
@@ -184,7 +187,9 @@ def _run_rollup(ctx, sources, config, output, map_output = None):
     # These deps are identified by the NodeModuleInfo provider.
     for d in ctx.attr.deps:
         if NodeModuleInfo in d:
-            direct_inputs += _filter_js_inputs(d.files)
+
+            # Note: we can't avoid calling .to_list() on files
+            direct_inputs += _filter_js_inputs(d.files.to_list())
 
     if ctx.file.license_banner:
         direct_inputs += [ctx.file.license_banner]
Original file line number	Diff line number	Diff line change
`@@ -42,7 +42,7 @@ def _sources_aspect_impl(target, ctx):`
`42`	`42`	`result = depset(transitive = [result, target.typescript.es5_sources])`
`43`	`43`	`elif hasattr(target, "files"):`
`44`	`44`	`result = depset(`
`45`		`- [f for f in target.files if f.path.endswith(".js")],`
	`45`	`+ [f for f in target.files.to_list() if f.path.endswith(".js")],`
`46`	`46`	`transitive = [result],`
`47`	`47`	`)`
`48`	`48`	`return struct(node_sources = result)`