Config option to change the default "node_modules" dirname

Summary: tldr: [Webpack has something similar to this option](https://webpack.github.io/docs/configuration.html#resolve-modulesdirectories) and some users find it useful.

This is a feature request from here: #915

Reviewed By: @gabelevi

Differential Revision: D2532941

fb-gh-sync-id: a7ae5f278345849b344863a68ecbc2baabc026d6

src/common/flowConfig.ml

@@ -43,6 +43,7 @@ module Opts = struct
     experimental_decorators: experimental_feature_mode;
     moduleSystem: moduleSystem;
     module_name_mappers: (Str.regexp * string) list;
+    node_resolver_dirnames: string list;
     munge_underscores: bool;
     module_file_exts: string list;
     suppress_comments: Str.regexp list;
@@ -100,6 +101,7 @@ module Opts = struct
     experimental_decorators = EXPERIMENTAL_WARN;
     moduleSystem = Node;
     module_name_mappers = [];
+    node_resolver_dirnames = ["node_modules"];
     munge_underscores = false;
     module_file_exts = [".js"; ".jsx";];
     suppress_comments = [];
@@ -530,6 +532,18 @@ let parse_options config lines = Opts.(
       });
     })
 
+    |> Opts.define_opt "module.system.node.resolve_dirname" Opts.({
+      _initializer = INIT_FN (fun opts -> {
+        opts with node_resolver_dirnames = [];
+      });
+      flags = [ALLOW_DUPLICATE];
+      optparser = optparse_string;
+      setter = (fun opts v ->
+        let node_resolver_dirnames = v :: opts.node_resolver_dirnames in
+        {opts with node_resolver_dirnames;}
+      );
+    })
+
     |> Opts.define_opt "munge_underscores" Opts.({
       _initializer = USE_DEFAULT;
       flags = [];

src/common/flowConfig.mli

@@ -18,6 +18,7 @@ module Opts : sig
     experimental_decorators: experimental_feature_mode;
     moduleSystem: moduleSystem;
     module_name_mappers: (Str.regexp * string) list;
+    node_resolver_dirnames: string list;
     munge_underscores: bool;
     module_file_exts: string list;
     suppress_comments: Str.regexp list;

src/typing/module_js.ml

@@ -309,31 +309,30 @@ module Node = struct
     let opts = get_config_options () in
     if Files_js.is_flow_file path
     then path_if_exists path
-    else
-      seq
-  (fun () ->
-         seqf
-           (fun ext -> path_if_exists (path ^ ext))
-           opts.FlowConfig.Opts.module_file_exts
-  )
-  (fun () ->
-   seq
-     (fun () ->
-      let package = Filename.concat path "package.json" in
-      parse_main package)
-     (fun () ->
-      let path = Filename.concat path "index.js" in
-      path_if_exists path
-     )
-  )
+    else seq
+      (fun () -> seqf
+        (fun ext -> path_if_exists (path ^ ext))
+        opts.FlowConfig.Opts.module_file_exts
+      )
+      (fun () -> seq
+        (fun () -> parse_main (Filename.concat path "package.json"))
+        (fun () -> path_if_exists (Filename.concat path "index.js"))
+      )
 
-  let rec node_module dir r = seq
-    (fun () -> resolve_relative dir (spf "node_modules%s%s" Filename.dir_sep r))
-    (fun () ->
-      let parent_dir = Filename.dirname dir in
-      if dir = parent_dir then None
-      else node_module (Filename.dirname dir) r
-    )
+  let rec node_module dir r =
+    let opts = get_config_options () in
+    seq
+      (fun () -> seqf
+        (resolve_relative dir)
+        (opts.FlowConfig.Opts.node_resolver_dirnames |> List.map (fun dirname ->
+          spf "%s%s%s" dirname Filename.dir_sep r
+        ))
+      )
+      (fun () ->
+        let parent_dir = Filename.dirname dir in
+        if dir = parent_dir then None
+        else node_module (Filename.dirname dir) r
+      )
 
   let relative r =
     Str.string_match Files_js.dir_sep r 0

tests/config_module_system_node_resolve_dirname/.flowconfig

@@ -0,0 +1,3 @@
+[options]
+module.system.node.resolve_dirname=node_modules
+module.system.node.resolve_dirname=custom_resolve_dir

tests/config_module_system_node_resolve_dirname/config_module_system_node_resolve_dirname.exp

@@ -0,0 +1,10 @@
+
+subdir/sublevel.js:5:2,5: string literal `subdir/custom_resolve_dir/testproj2`
+Expected string literal `node_modules/testproj2`, got `subdir/custom_resolve_dir/testproj2` instead
+subdir/sublevel.js:5:8,31: string literal `node_modules/testproj2`
+
+toplevel.js:6:2,5: string literal `node_modules/testproj`
+Expected string literal `custom_resolve_dir/testproj`, got `node_modules/testproj` instead
+toplevel.js:6:8,36: string literal `custom_resolve_dir/testproj`
+
+Found 2 errors

tests/config_module_system_node_resolve_dirname/custom_resolve_dir/testproj/index.js

@@ -0,0 +1,3 @@
+// @flow
+
+export var name: "otherdir/testproj" = "otherdir/testproj";

tests/config_module_system_node_resolve_dirname/custom_resolve_dir/testproj2/index.js

@@ -0,0 +1,3 @@
+// @flow
+
+export var name: "otherdir/testproj2" = "otherdir/testproj2";

tests/config_module_system_node_resolve_dirname/subdir/custom_resolve_dir/testproj2/index.js

@@ -0,0 +1,3 @@
+// @flow
+
+export var name: "subdir/custom_resolve_dir/testproj2" = "subdir/custom_resolve_dir/testproj2";

tests/config_module_system_node_resolve_dirname/subdir/sublevel.js

@@ -0,0 +1,6 @@
+// @flow
+
+import {name} from "testproj2";
+
+(name: "node_modules/testproj2"); // Error: Resolve from sibling 'custom_resolve_dir' first!
+(name: "subdir/custom_resolve_dir/testproj2");

tests/config_module_system_node_resolve_dirname/toplevel.js

@@ -0,0 +1,6 @@
+// @flow
+
+import {name} from "testproj";
+
+(name: "node_modules/testproj");
+(name: "custom_resolve_dir/testproj"); // Error: Resolve from node_modules first!