Implement proposal-top-level-await

README.md

@@ -99,6 +99,8 @@ object referring to that same position.
 - **allowImportExportEverywhere**: By default, `import` and `export`
   declarations can only appear at a program's top level. Setting this
   option to `true` allows them anywhere where a statement is allowed.
+
+- **allowAwaitOutsideFunction**: By default, `await` expressions can only appear inside `async` functions. Setting this option to `true` allows to have top-level `await` expressions. They are still not allowed in non-`async` functions, though.
 
 - **allowHashBang**: When this is enabled (off by default), if the
   code starts with the characters `#!` (as in a shellscript), the

src/expression.js

@@ -198,7 +198,7 @@ pp.buildBinary = function(startPos, startLoc, left, right, op, logical) {
 
 pp.parseMaybeUnary = function(refDestructuringErrors, sawUnary) {
   let startPos = this.start, startLoc = this.startLoc, expr
-  if (this.inAsync && this.isContextual("await")) {
+  if (this.isContextual("await") && (this.inAsync || (!this.inFunction && this.options.allowAwaitOutsideFunction))) {
     expr = this.parseAwait()
     sawUnary = true
   } else if (this.type.prefix) {

src/loose/expression.js

@@ -111,7 +111,9 @@ lp.parseExprOp = function(left, start, minPrec, noIn, indent, line) {
 
 lp.parseMaybeUnary = function(sawUnary) {
   let start = this.storeCurrentPos(), expr
-  if (this.options.ecmaVersion >= 8 && this.inAsync && this.toks.isContextual("await")) {
+  if (this.options.ecmaVersion >= 8 && this.toks.isContextual("await") &&
+    (this.inAsync || (!this.inFunction && this.options.allowAwaitOutsideFunction))
+  ) {
     expr = this.parseAwait()
     sawUnary = true
   } else if (this.tok.type.prefix) {
@@ -514,26 +516,29 @@ lp.parseFunctionParams = function(params) {
 }
 
 lp.parseMethod = function(isGenerator, isAsync) {
-  let node = this.startNode(), oldInAsync = this.inAsync
+  let node = this.startNode(), oldInAsync = this.inAsync, oldInFunction = this.inFunction
   this.initFunction(node)
   if (this.options.ecmaVersion >= 6)
     node.generator = !!isGenerator
   if (this.options.ecmaVersion >= 8)
     node.async = !!isAsync
   this.inAsync = node.async
+  this.inFunction = true
   node.params = this.parseFunctionParams()
   node.body = this.parseBlock()
   this.toks.adaptDirectivePrologue(node.body.body)
   this.inAsync = oldInAsync
+  this.inFunction = oldInFunction
   return this.finishNode(node, "FunctionExpression")
 }
 
 lp.parseArrowExpression = function(node, params, isAsync) {
-  let oldInAsync = this.inAsync
+  let oldInAsync = this.inAsync, oldInFunction = this.inFunction
   this.initFunction(node)
   if (this.options.ecmaVersion >= 8)
     node.async = !!isAsync
   this.inAsync = node.async
+  this.inFunction = true
   node.params = this.toAssignableList(params, true)
   node.expression = this.tok.type !== tt.braceL
   if (node.expression) {
@@ -543,6 +548,7 @@ lp.parseArrowExpression = function(node, params, isAsync) {
     this.toks.adaptDirectivePrologue(node.body.body)
   }
   this.inAsync = oldInAsync
+  this.inFunction = oldInFunction
   return this.finishNode(node, "ArrowFunctionExpression")
 }
 

src/loose/state.js

@@ -23,6 +23,7 @@ export class LooseParser {
     this.curLineStart = 0
     this.nextLineStart = this.lineEnd(this.curLineStart) + 1
     this.inAsync = false
+    this.inFunction = false
     // Load plugins
     this.options.pluginsLoose = options.pluginsLoose || {}
     this.loadPlugins(this.options.pluginsLoose)

src/loose/statement.js

@@ -331,7 +331,7 @@ lp.parseClass = function(isStatement) {
 }
 
 lp.parseFunction = function(node, isStatement, isAsync) {
-  let oldInAsync = this.inAsync
+  let oldInAsync = this.inAsync, oldInFunction = this.inFunction
   this.initFunction(node)
   if (this.options.ecmaVersion >= 6) {
     node.generator = this.eat(tt.star)
@@ -342,10 +342,12 @@ lp.parseFunction = function(node, isStatement, isAsync) {
   if (this.tok.type === tt.name) node.id = this.parseIdent()
   else if (isStatement === true) node.id = this.dummyIdent()
   this.inAsync = node.async
+  this.inFunction = true
   node.params = this.parseFunctionParams()
   node.body = this.parseBlock()
   this.toks.adaptDirectivePrologue(node.body.body)
   this.inAsync = oldInAsync
+  this.inFunction = oldInFunction
   return this.finishNode(node, isStatement ? "FunctionDeclaration" : "FunctionExpression")
 }
 

src/options.js

@@ -34,6 +34,9 @@ export const defaultOptions = {
   // When enabled, import/export statements are not constrained to
   // appearing at the top of the program.
   allowImportExportEverywhere: false,
+  // When enabled, await identifiers are allowed to appear at the top-level scope,
+  // but they are still not allowed in non-async functions.
+  allowAwaitOutsideFunction: false,
   // When enabled, hashbang directive in the beginning of file
   // is allowed and treated as a line comment.
   allowHashBang: false,

test/run.js

@@ -7,6 +7,7 @@
     require("./tests-harmony.js");
     require("./tests-es7.js");
     require("./tests-asyncawait.js");
+    require("./tests-await-top-level.js");
     require("./tests-trailing-commas-in-func.js");
     require("./tests-template-literal-revision.js");
     require("./tests-directive.js");

test/tests-await-top-level.js

@@ -0,0 +1,38 @@
+if (typeof exports != "undefined") {
+  var test = require("./driver.js").test
+  var testFail = require("./driver.js").testFail
+}
+
+//------------------------------------------------------------------------------
+// await-top-level
+//------------------------------------------------------------------------------
+
+testFail("await 1", "Unexpected token (1:6)", {ecmaVersion: 8})
+test("await 1", {
+  "type": "Program",
+  "start": 0,
+  "end": 7,
+  "body": [
+    {
+      "type": "ExpressionStatement",
+      "start": 0,
+      "end": 7,
+      "expression": {
+        "type": "AwaitExpression",
+        "start": 0,
+        "end": 7,
+        "argument": {
+          "type": "Literal",
+          "start": 6,
+          "end": 7,
+          "value": 1
+        }
+      }
+    }
+  ]
+}, {allowAwaitOutsideFunction: true, ecmaVersion: 8})
+testFail("function foo() {return await 1}", "Unexpected token (1:29)", {ecmaVersion: 8})
+testFail("function foo() {return await 1}", "Unexpected token (1:29)", {
+  allowAwaitOutsideFunction: true,
+  ecmaVersion: 8
+})