support top-level await (#5487)

README.md

@@ -519,7 +519,8 @@ if (result.error) throw result.error;
     Pass an object to specify custom [mangle property options](#mangle-properties-options).
 
 - `module` (default: `false`) — set to `true` if you wish to process input as
-  ES module, i.e. implicit `"use strict";` alongside with `toplevel` enabled.
+  ES module, i.e. implicit `"use strict";` and support for top-level `await`,
+  alongside with `toplevel` enabled.
 
 - `nameCache` (default: `null`) — pass an empty object `{}` or a previously
   used `nameCache` object if you wish to cache mangled variable and
@@ -632,7 +633,13 @@ to be `false` and all symbol names will be omitted.
 
 - `bare_returns` (default: `false`) — support top level `return` statements
 
-- `html5_comments` (default: `true`)
+- `expression` (default: `false`) — parse as a single expression, e.g. JSON
+
+- `html5_comments` (default: `true`) — process HTML comment as workaround for
+  browsers which do not recognise `<script>` tags
+
+- `module` (default: `false`) — set to `true` if you wish to process input as
+  ES module, i.e. implicit `"use strict";` and support for top-level `await`.
 
 - `shebang` (default: `true`) — support `#!command` as the first line
 
@@ -1415,9 +1422,10 @@ To allow for better optimizations, the compiler makes various assumptions:
       function f() {
           throw 42;
       }
-  } catch (e) {}
+  } catch (e) {
-  console.log(typeof f);
+      console.log(typeof f, e);
-  // Expected: "function"
+  }
-  // Actual:   "undefined"
+  // Expected: "function 42"
+  // Actual:   "undefined 42"
   ```
   UglifyJS may modify the input which in turn may suppress those errors.

lib/parse.js

@@ -2541,7 +2541,10 @@ function parse($TEXT, options) {
     return function() {
         var start = S.token;
         var body = [];
-        if (options.module) S.input.add_directive("use strict");
+        if (options.module) {
+            S.in_async = true;
+            S.input.add_directive("use strict");
+        }
         S.input.push_directives_stack();
         while (!is("eof"))
             body.push(statement());

test/ufuzz/index.js

@@ -405,7 +405,7 @@ function createTopLevelCode() {
     unique_vars.length = 0;
     classes.length = 0;
     allow_this = true;
-    async = false;
+    async = SUPPORT.async && rng(200) == 0;
     has_await = false;
     export_default = false;
     generator = false;
@@ -413,7 +413,7 @@ function createTopLevelCode() {
     funcs = 0;
     clazz = 0;
     imports = 0;
-    in_class = 0;
+    in_class = async;
     called = Object.create(null);
     var s = [
         strictMode(),
@@ -2087,6 +2087,7 @@ if (require.main !== module) {
 }
 
 function run_code(code, toplevel, timeout) {
+    if (async && has_await) code = "(async function(){\n" + code + "\n})();";
     return sandbox.run_code(sandbox.patch_module_statements(code), toplevel, timeout);
 }
 
@@ -2502,6 +2503,7 @@ for (var round = 1; round <= num_iterations; round++) {
     minify_options.forEach(function(options) {
         var o = JSON.parse(options);
         var toplevel = sandbox.has_toplevel(o);
+        if (async && has_await) o.parse = { module: true };
         o.validate = true;
         uglify_code = UglifyJS.minify(original_code, o);
         original_result = orig_result[toplevel ? 1 : 0];