Make moment optional from our UMD builds

Create a rollup plugin altering the UMD header to wrap optional dependencies between try/catch, which allows to load moment only when the dependency is installed.

Since AMD loaders are asynchronous, `'moment'` needs to be explicitly loaded before 'chart.js' so when 'chart.js' requires moment, it's already loaded and returns synchronously (at least with requirejs).

rollup.config.js

@@ -3,6 +3,7 @@
 const commonjs = require('rollup-plugin-commonjs');
 const resolve = require('rollup-plugin-node-resolve');
 const terser = require('rollup-plugin-terser').terser;
+const optional = require('./rollup.plugins').optional;
 const pkg = require('./package.json');
 
 const input = 'src/chart.js';
@@ -21,7 +22,10 @@ module.exports = [
 		input: input,
 		plugins: [
 			resolve(),
-			commonjs()
+			commonjs(),
+			optional({
+				include: ['moment']
+			})
 		],
 		output: {
 			name: 'Chart',
@@ -42,6 +46,9 @@ module.exports = [
 		plugins: [
 			resolve(),
 			commonjs(),
+			optional({
+				include: ['moment']
+			}),
 			terser({
 				output: {
 					preamble: banner

rollup.plugins.js

@@ -0,0 +1,61 @@
+/* eslint-env es6 */
+
+const UMD_WRAPPER_RE = /(\(function \(global, factory\) \{)((?:\s.*?)*)(\}\(this,)/;
+const CJS_FACTORY_RE = /(module.exports = )(factory\(.*?\))( :)/;
+const AMD_FACTORY_RE = /(define\()(.*?, factory)(\) :)/;
+
+function optional(config = {}) {
+	return {
+		name: 'optional',
+		renderChunk(code, chunk, options) {
+			if (options.format !== 'umd') {
+				this.error('only UMD format is currently supported');
+			}
+
+			const wrapper = UMD_WRAPPER_RE.exec(code);
+			const include = config.include;
+			if (!wrapper) {
+				this.error('failed to parse the UMD wrapper');
+			}
+
+			let content = wrapper[2];
+			let factory = (CJS_FACTORY_RE.exec(content) || [])[2];
+			let updated = false;
+
+			for (let lib of chunk.imports) {
+				if (!include || include.indexOf(lib) !== -1) {
+					const regex = new RegExp(`require\\('${lib}'\\)`);
+					if (!regex.test(factory)) {
+						this.error(`failed to parse the CJS require for ${lib}`);
+					}
+
+					// We need to write inline try / catch with explicit require
+					// in order to enable statical extraction of dependencies:
+					// try { return require('moment'); } catch(e) {}
+					const loader = `function() { try { return require('${lib}'); } catch(e) { } }()`;
+					factory = factory.replace(regex, loader);
+					updated = true;
+				}
+			}
+
+			if (!updated) {
+				return;
+			}
+
+			// Replace the CJS factory by our updated one.
+			content = content.replace(CJS_FACTORY_RE, `$1${factory}$3`);
+
+			// Replace the AMD factory by our updated one: we need to use the
+			// following AMD form in order to be able to try/catch require:
+			// define(['require'], function(require) { ... require(...); ... })
+			// https://github.com/amdjs/amdjs-api/wiki/AMD#using-require-and-exports
+			content = content.replace(AMD_FACTORY_RE, `$1['require'], function(require) { return ${factory}; }$3`);
+
+			return code.replace(UMD_WRAPPER_RE, `$1${content}$3`);
+		}
+	};
+}
+
+module.exports = {
+	optional
+};