feat: allow fallback if data for target not available (with option to suppress)

.gitignore

@@ -1,5 +1,5 @@
 .DS_Store
 node_modules
-dist
+# dist
 test/**/actual
 sandbox

src/index.js

@@ -12,29 +12,42 @@ const { parse } = [acornObjectSpread, acornJsx].reduce(
 
 const dangerousTransforms = ['dangerousTaggedTemplateString', 'dangerousForOf'];
 
+const fallbackOrThrow = (fallback, environment, version) => {
+	const error = `Unknown environment/version '${environment}/${version}'. Please raise an issue at https://github.com/Rich-Harris/buble/issues`
+	if (!('bitmask' in fallback)) throw new Error(error)
+	if (!fallback.suppress) console.error(error)
+	return fallback.bitmask
+}
+
+const nearest = (versions, version, fallback, environment) => {
+	const nearestVersion = Object.keys(versions).sort().reverse().find(v => v <= version)
+	return nearestVersion
+		? versions[nearestVersion]
+		: fallbackOrThrow(fallback, environment, version)
+}
+
 export function target(target) {
+	const fallback = {} 
+	if ('fallback' in target) {
+		fallback.environment = Object.keys(target.fallback).filter(d => d != 'suppress').pop()
+		fallback.version = target.fallback[fallback.environment]
+		fallback.suppress = !!target.fallback.suppress
+		delete target.fallback
+		if (matrix[fallback.environment] && (fallback.version in matrix[fallback.environment]))
+			fallback.bitmask = matrix[fallback.environment][fallback.version]
+	}
+
 	const targets = Object.keys(target);
 	let bitmask = targets.length
 		? 0b11111111111111111111111111111111
 		: 0b01000000000000000000000000000000;
 
 	Object.keys(target).forEach(environment => {
-		const versions = matrix[environment];
-		if (!versions)
-			throw new Error(
-				`Unknown environment '${environment}'. Please raise an issue at https://github.com/Rich-Harris/buble/issues`
-			);
-
-		const targetVersion = target[environment];
-		if (!(targetVersion in versions))
-			throw new Error(
-				`Support data exists for the following versions of ${environment}: ${Object.keys(
-					versions
-				).join(
-					', '
-				)}. Please raise an issue at https://github.com/Rich-Harris/buble/issues`
-			);
-		const support = versions[targetVersion];
+		const versions = matrix[environment]
+				, version  = target[environment]
+				, support  = !versions           ? fallbackOrThrow(fallback, environment, version)
+									 : version in versions ? versions[version]
+																				 : nearest(versions, version, fallback, environment)
 
 		bitmask &= support;
 	});

test/test.js

@@ -182,6 +182,27 @@ describe('buble', () => {
 			assert.ok(!transforms.arrow);
 		});
 
+		it('falls back if nonexistent browser', () => {
+			var transforms = buble.target({ nonexistent: 99, fallback: { node: 5 }});
+
+			assert.ok(transforms.defaultParameter);
+			assert.ok(!transforms.arrow);
+		});
+
+		it('uses nearest version if nonexistent version', () => {
+			var transforms = buble.target({ chrome: 51 });
+			assert.ok(transforms.exponentiation); // chrome 51
+
+			var transforms = buble.target({ chrome: 99 });
+			assert.ok(!transforms.exponentiation); // chrome 52
+
+			var transforms = buble.target({ chrome: 99, fallback: { node: 5 } });
+			assert.ok(!transforms.exponentiation); // chrome 52
+
+			var transforms = buble.target({ chrome: 10, fallback: { node: 5 }});
+			assert.ok(!transforms.arrow); // node 5
+		});
+
 		it('only applies necessary transforms', () => {
 			var source = `
 				const power = ( base, exponent = 2 ) => Math.pow( base, exponent );`;