Fix rest parameters indexing with TypeScript 'this parameter' (#9714)

* Fix rest parameters indexing with TypeScript 'this parameter'

The TypeScript [this parameter][0] is a fake parameter and should not be
taken into account when counting the function parameters.  This patch
skips it by using the condition taken from the `transform-typescript`
plugin.

Note: since the `transform-typescript` plugin is removing this kind of
parameter, including it before the `transform-parameters` plugin solves
the issue.  This patch fixes the issue in other cases.

[0]: https://www.typescriptlang.org/docs/handbook/functions.html#this-parameters

* nit: improve the 'this parameter' detection condition

* improve performance by checking the parameter list length before
accessing it
* simplify the test a bit by using the `isIdentifier` second
parameter

packages/babel-plugin-transform-parameters/src/rest.js

@@ -155,6 +155,16 @@ const memberExpressionOptimisationVisitor = {
     }
   },
 };
+
+function getParamsCount(node) {
+  let count = node.params.length;
+  // skip the first parameter if it is a TypeScript 'this parameter'
+  if (count > 0 && t.isIdentifier(node.params[0], { name: "this" })) {
+    count -= 1;
+  }
+  return count;
+}
+
 function hasRest(node) {
   const length = node.params.length;
   return length > 0 && t.isRestElement(node.params[length - 1]);
@@ -244,10 +254,12 @@ export default function convertFunctionRest(path) {
     node.body.body.unshift(declar);
   }
 
+  const paramsCount = getParamsCount(node);
+
   // check and optimise for extremely common cases
   const state = {
     references: [],
-    offset: node.params.length,
+    offset: paramsCount,
 
     argumentsNode: argsId,
     outerBinding: scope.getBindingIdentifier(rest.name),
@@ -297,12 +309,12 @@ export default function convertFunctionRest(path) {
     state.candidates.map(({ path }) => path),
   );
 
-  const start = t.numericLiteral(node.params.length);
+  const start = t.numericLiteral(paramsCount);
   const key = scope.generateUidIdentifier("key");
   const len = scope.generateUidIdentifier("len");
 
   let arrKey, arrLen;
-  if (node.params.length) {
+  if (paramsCount) {
     // this method has additional params, so we need to subtract
     // the index of the current argument position from the
     // position in the array that we want to populate

packages/babel-plugin-transform-parameters/test/fixtures/parameters/options.json

@@ -2,6 +2,7 @@
   "plugins": [
     "proposal-class-properties",
     "external-helpers",
+    "syntax-typescript",
     "syntax-flow",
     "transform-parameters",
     "transform-block-scoping",

packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-ts-this-parameter/input.js

@@ -0,0 +1,7 @@
+function u(this: Foo, ...items) {
+  items[0];
+}
+
+function v(this: Foo, event: string, ...args: any[]) {
+  args;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/rest-ts-this-parameter/output.js

@@ -0,0 +1,11 @@
+function u(this: Foo) {
+  arguments.length <= 0 ? undefined : arguments[0];
+}
+
+function v(this: Foo, event: string) {
+  for (var _len = arguments.length, args = new Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
+    args[_key - 1] = arguments[_key];
+  }
+
+  args;
+}