Support top-level await (#5371)

* Support top-level await

* Remove code duplication

* Avoid use of trimEnd so tests pass in old Node

* Proposed rewrite of tests

* startsWith tests; revert eqJS

* build:browser

src/nodes.coffee

@@ -513,6 +513,8 @@ exports.Root = class Root extends Base
   constructor: (@body) ->
     super()
 
+    @isAsync = (new Code [], @body).isAsync
+
   children: ['body']
 
   # Wrap everything in a safety closure, unless requested not to. It would be
@@ -525,7 +527,13 @@ exports.Root = class Root extends Base
     @initializeScope o
     fragments = @body.compileRoot o
     return fragments if o.bare
-    [].concat @makeCode("(function() {\n"), fragments, @makeCode("\n}).call(this);\n")
+    parts = []
+    parts.push @makeCode '('
+    parts.push @makeCode 'async ' if @isAsync
+    parts.push @makeCode 'function() {\n'
+    parts.push ...fragments
+    parts.push @makeCode '\n}).call(this);\n'
+    [].concat ...parts
 
   initializeScope: (o) ->
     o.scope = new Scope null, @body, null, o.referencedVars ? []
@@ -4782,7 +4790,7 @@ exports.Op = class Op extends Base
   compileContinuation: (o) ->
     parts = []
     op = @operator
-    @checkContinuation o
+    @checkContinuation o unless @isAwait()
     if 'expression' in Object.keys(@first) and not (@first instanceof Throw)
       parts.push @first.expression.compileToFragments o, LEVEL_OP if @first.expression?
     else
@@ -4817,7 +4825,7 @@ exports.Op = class Op extends Base
       @error 'delete operand may not be argument or var'
 
   astNode: (o) ->
-    @checkContinuation o if @isYield() or @isAwait()
+    @checkContinuation o if @isYield()
     @checkDeleteOperand o
     super o
 

test/async.coffee

@@ -204,3 +204,17 @@ test "#3199: await multiline implicit object", ->
         type: 'a'
         msg: 'b'
     eq undefined, y
+
+test "top-level await", ->
+  eqJS 'await null', 'await null;'
+
+test "top-level wrapper has correct async attribute", ->
+  starts = (code, prefix) ->
+    compiled = CoffeeScript.compile code
+    unless compiled.startsWith prefix
+      fail """Expected generated JavaScript to start with:
+        #{reset}#{prefix}#{red}
+        but instead it was:
+        #{reset}#{compiled}#{red}"""
+  starts 'await null', '(async function'
+  starts 'do -> await null', '(function'

test/error_messages.coffee

@@ -1257,14 +1257,7 @@ test "CoffeeScript keywords cannot be used as local names in import list aliases
                     ^^^^^^
   '''
 
-test "cannot have `await` outside a function", ->
-  assertErrorFormat '''
-    await 1
-  ''', '''
-    [stdin]:1:1: error: await can only occur inside functions
-    await 1
-    ^^^^^^^
-  '''
+test "cannot have `await return` outside a function", ->
   assertErrorFormat '''
     await return
   ''', '''