feat: Make UMD build be compatible with IE11

Fixes #79

package.json

@@ -35,7 +35,7 @@
     "clean": "npm run clean:lib && npm run clean:es && npm run clean:dist",
     "build:lib": "npm run clean:lib && rollup -c -o lib/index.js -f cjs --environment declaration",
     "build:es": "npm run clean:es && rollup -c -o es/index.js -f es",
-    "build:dist": "npm run clean:dist && rollup -c -o dist/index.js -f umd --name squarify --environment minify",
+    "build:dist": "npm run clean:dist && rollup -c -o dist/index.js -f umd --name squarify --environment minify --environment es5",
     "build": "npm run clean && npm run typecheck && npm run build:lib && npm run build:dist && npm run build:es && npm run generate:typedef",
     "tslint": "tslint --project tsconfig.json --config tslint.json",
     "tslint:fix": "tslint --project tsconfig.json --config tslint.json --fix",

rollup.config.js

@@ -4,12 +4,33 @@ import resolve from 'rollup-plugin-node-resolve';
 
 const shouldMinify = !!process.env.minify;
 
+
+const commonBabelPluginOptions = {
+  exclude: 'node_modules/**',
+  extensions: ['.ts'],
+};
+let babelPlugin;
+if (process.env.es5) {
+  // Make UMD build (`dist`) be compatible with IE 11.
+  babelPlugin = babel({
+    ...commonBabelPluginOptions,
+    babelrc: false,
+    presets: [
+      ['@babel/preset-env', {
+        'targets': 'ie 11',
+      },
+      ],
+      '@babel/preset-typescript',
+    ],
+  });
+} else {
+  // Otherwise,
+  babelPlugin = babel(commonBabelPluginOptions);
+}
+
 let plugins = [
   resolve(),
-  babel({
-    exclude: 'node_modules/**',
-    extensions: ['.ts'],
-  }),
+  babelPlugin,
 ];
 
 if (shouldMinify) {