Add option to block-scoping to slow on throw code (#5236)

The let/const plugin can add closures where you don't expect them. This is undesirable in some perf-sensitive projects (ex: React). I added an option that throws whenever the plugin adds a function (as opposed to simply renaming variables when converting to var).

packages/babel-plugin-transform-es2015-block-scoping/README.md

@@ -14,12 +14,26 @@ npm install --save-dev babel-plugin-transform-es2015-block-scoping
 
 **.babelrc**
 
+Without options:
+
 ```json
 {
   "plugins": ["transform-es2015-block-scoping"]
 }
 ```
 
+With options:
+
+```json
+{
+  "plugins": [
+    ["transform-es2015-block-scoping", {
+      "throwIfClosureRequired": true
+    }]
+  ]
+}
+```
+
 ### Via CLI
 
 ```sh
@@ -33,3 +47,15 @@ require("babel-core").transform("code", {
   plugins: ["transform-es2015-block-scoping"]
 });
 ```
+
+## Options `throwIfClosureRequired`
+
+In cases such as the following it's impossible to rewrite let/const without adding an additional function and closure while transforming:
+
+```javascript
+for (let i = 0; i < 5; i++) {
+  setTimeout(() => console.log(i), 1);
+}
+```
+
+In extremely performance-sensitive code, this can be undesirable. If `"throwIfClosureRequired": true` is set, Babel throws when transforming these patterns instead of automatically adding an additional function.

packages/babel-plugin-transform-es2015-block-scoping/src/index.js

@@ -360,6 +360,12 @@ class BlockScoping {
   }
 
   wrapClosure() {
+    if (this.file.opts.throwIfClosureRequired) {
+      throw this.blockPath.buildCodeFrameError(
+        "Compiling let/const in this block would add a closure " +
+        "(throwIfClosureRequired)."
+      );
+    }
     const block = this.block;
 
     const outsideRefs = this.outsideLetReferences;

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/for-const-closure/actual.js

@@ -0,0 +1,6 @@
+for (let i = 0; i < 5; i++) {
+  const l = i;
+  setTimeout(function() {
+    console.log(l);
+  }, 1);
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/for-const-closure/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Compiling let/const in this block would add a closure (throwIfClosureRequired)."
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/function/actual.js

@@ -0,0 +1,3 @@
+function test() {
+  let foo = "bar";
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/function/expected.js

@@ -0,0 +1,3 @@
+function test() {
+  var foo = "bar";
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": [["transform-es2015-block-scoping", { "throwIfClosureRequired": true }], "syntax-jsx", "transform-react-jsx", "transform-es2015-block-scoped-functions", "transform-es2015-arrow-functions"]
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/superswitch/actual.js

@@ -0,0 +1,16 @@
+function foo() {
+  switch (2) {
+    case 0: {
+      if (true) {
+        return;
+      }
+
+      const stuff = new Map();
+      const data = 0;
+      stuff.forEach(() => {
+        const d = data;
+      });
+      break;
+    }
+  }
+}

packages/babel-plugin-transform-es2015-block-scoping/test/fixtures/throwIfClosureRequired/superswitch/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Compiling let/const in this block would add a closure (throwIfClosureRequired)."
+}