add const-to-var plugin

Author: lobsterkatie

rollup/npmHelpers.js

@@ -7,7 +7,7 @@ import * as path from 'path';
 
 import deepMerge from 'deepmerge';
 
-import { makeNodeResolvePlugin, makeSucrasePlugin } from './plugins/index.js';
+import { makeConstToVarPlugin, makeNodeResolvePlugin, makeSucrasePlugin } from './plugins/index.js';
 
 const packageDotJSON = require(path.resolve(process.cwd(), './package.json'));
 
@@ -21,6 +21,7 @@ export function makeBaseNPMConfig(options = {}) {
 
   const nodeResolvePlugin = makeNodeResolvePlugin();
   const sucrasePlugin = makeSucrasePlugin();
+  const constToVarPlugin = makeConstToVarPlugin();
 
   return {
     input: entrypoints,
@@ -61,7 +62,7 @@ export function makeBaseNPMConfig(options = {}) {
       interop: esModuleInterop ? 'auto' : 'esModule',
     },
 
-    plugins: [nodeResolvePlugin, sucrasePlugin],
+    plugins: [nodeResolvePlugin, sucrasePlugin, constToVarPlugin],
 
     // don't include imported modules from outside the package in the final output
     external: [

rollup/plugins/npmPlugins.js

@@ -1,7 +1,9 @@
 /**
+ * Replace plugin docs: https://github.com/rollup/plugins/tree/master/packages/replace
  * Sucrase plugin docs: https://github.com/rollup/plugins/tree/master/packages/sucrase
  */
 
+import replace from '@rollup/plugin-replace';
 import sucrase from '@rollup/plugin-sucrase';
 
 /**
@@ -14,3 +16,29 @@ export function makeSucrasePlugin() {
     transforms: ['typescript', 'jsx'],
   });
 }
+
+/**
+ * Create a plugin to switch all instances of `const` to `var`, both to prevent problems when we shadow `global` and
+ * because it's fewer characters.
+ *
+ * Note that the generated plugin runs the replacement both before and after rollup does its code manipulation, to
+ * increase the chances that nothing is missed.
+ *
+ * TODO This is pretty brute-force-y. Perhaps we could switch to using a parser, the way we (will) do for both our jest
+ * transformer and the polyfill build script.
+ *
+ * @returns An instance of the `@rollup/plugin-replace` plugin
+ */
+export function makeConstToVarPlugin() {
+  return replace({
+    // TODO `preventAssignment` will default to true in version 5.x of the replace plugin, at which point we can get rid
+    // of this. (It actually makes no difference in this case whether it's true or false, since we never assign to
+    // `const`, but if we don't give it a value, it will spam with warnings.)
+    preventAssignment: true,
+    values: {
+      // Include a space at the end to guarantee we're not accidentally catching the beginning of the words "constant,"
+      // "constantly," etc.
+      'const ': 'var ',
+    },
+  });
+}