Support multiline ES6 `export ... from` syntax

[au-phiware]

Description of problem

When parsing ES6 code of the form export ... from, the compiler fails to generate the correct goog.require statement.

Steps to reproduce

Given the following files:

//greet.js
export default function greet(m) {
    document.write("\nHello, " + m);
};

//hello.js
export {
    default as helloGreet
} from "./greet";

Execute:

java -jar $JAR -O BUNDLE --formatting PRETTY_PRINT --module_resolution NODE --js_output_file=/dev/stdout -W VERBOSE --debug --dependency_mode STRICT --entry_point hello.js greet.js hello.js | grep -A 11 -E 'greet|hello'

Expected outcome

Closure compiler should detect greet.js as a dependency of hello.js and output them both under strict dependency mode.

//greet.js
$jscomp.registerAndLoadModule(function($$require, $$exports, $$module) {
  "use strict";
  Object.defineProperties($$exports, {"default":{enumerable:true, get:function() {
    return greet;
  }}});
  function greet(m) {
    document.write("\nHello, " + m);
  }
}, "greet.js", []);

//hello.js
$jscomp.registerAndLoadModule(function($$require, $$exports, $$module) {
  "use strict";
  Object.defineProperties($$exports, {helloGreet:{enumerable:true, get:function() {
    return module$greet["default"];
  }}});
  var module$greet = $$require("greet.js");
}, "hello.js", ["greet.js"]);

Actual outcome

JsFileParser fails compute the list of requires.

//hello.js
$jscomp.registerAndLoadModule(function($$require, $$exports, $$module) {
  "use strict";
  Object.defineProperties($$exports, {helloGreet:{enumerable:true, get:function() {
    return module$greet["default"];
  }}});
  var module$greet = $$require("greet.js");
}, "hello.js", ["greet.js"]);

Workaround/patches

Condensing the export to a single line works around the issue but may be infeasible for third party libs (e.g. from NPM).

Avoiding JsFileParser also fixes the problem:

diff --git a/src/com/google/javascript/jscomp/CompilerInput.java b/src/com/google/javascript/jscomp/CompilerInput.java
index 62609562b..4e4d3e3dc 100644
--- a/src/com/google/javascript/jscomp/CompilerInput.java
+++ b/src/com/google/javascript/jscomp/CompilerInput.java
@@ -285,7 +285,7 @@ public class CompilerInput extends DependencyInfo.Base implements SourceAst {
 
     // If the code is a JsAst, then it was originally JS code, and is compatible with the
     // regex-based parsing of JsFileParser.
-    if (ast instanceof JsAst && JsFileParser.isSupported()) {
+    if (false && ast instanceof JsAst && JsFileParser.isSupported()) {
       // Look at the source code.
       // Note: it's OK to use getName() instead of
       // getPathRelativeToClosureBase() here because we're not using

Analysis

Due to the line parsing nature of JsFileParser, it fails to look ahead when faced with a line that starts with export (or import) but doesn't contain a semicolon. In fact, under shortcut mode, it appears to halt the parser.

[au-phiware]

Also, there is no output using -O ADVANCED... but that could just be me not understanding how to use the compiler.

[brad4d]

@jplaisted I think I've heard you mention this export ... from syntax before, but I didn't think we supported it.
Do you know what the status of this feature is?

[jplaisted]

We support export {name} from but not export * from. However neither has anything to do with goog.require.

The issue here is that JsFileParser is regex based and doesn't detect multiline import or export statements, as @au-phiware mentoined.

@au-phiware fyi the output you've pasted is the "bundled" output. A "bundle" is the minimal transformations needed to get all input files in one output file. ES6 modules can't be bundled so we tranform them to CommonJS-like modules (the above output). That output is not meant to be fed back into the compiler. It also doesn't emit goog.requires to avoid any dependency on Closure Library.

[jplaisted]

FYI @shicks

[jplaisted]

Sorry, regex is a red herring. It just happens to parse files line by line.