allow options.external to be a function. closes #522

src/Bundle.js

@@ -34,7 +34,7 @@ export default class Bundle {
 		this.treeshake = options.treeshake !== false;
 
 		this.resolveId = first(
-			[ id => ~this.external.indexOf( id ) ? false : null ]
+			[ id => this.isExternal( id ) ? false : null ]
 				.concat( this.plugins.map( plugin => plugin.resolveId ).filter( Boolean ) )
 				.concat( resolveId )
 		);
@@ -62,7 +62,13 @@ export default class Bundle {
 
 		this.assumedGlobals = blank();
 
-		this.external = ensureArray( options.external ).map( id => id.replace( /[\/\\]/g, '/' ) );
+		if ( typeof options.external === 'function' ) {
+			this.isExternal = options.external;
+		} else {
+			const ids = ensureArray( options.external ).map( id => id.replace( /[\/\\]/g, '/' ) );
+			this.isExternal = id => ids.indexOf( id ) !== -1;
+		}
+
 		this.onwarn = options.onwarn || makeOnwarn();
 
 		// TODO strictly speaking, this only applies with non-ES6, non-default-only bundles
@@ -198,14 +204,14 @@ export default class Bundle {
 						// This could be an external, relative dependency, based on the current module's parent dir.
 						externalName = resolve( module.id, '..', source );
 					}
-					const forcedExternal = externalName && ~this.external.indexOf( externalName );
+					const forcedExternal = externalName && this.isExternal( externalName );
 
 					if ( !resolvedId || forcedExternal ) {
 						let normalizedExternal = source;
 
 						if ( !forcedExternal ) {
 							if ( isRelative( source ) ) throw new Error( `Could not resolve ${source} from ${module.id}` );
-							if ( !~this.external.indexOf( source ) ) this.onwarn( `Treating '${source}' as external dependency` );
+							if ( !this.isExternal( source ) ) this.onwarn( `Treating '${source}' as external dependency` );
 						} else if ( resolvedId ) {
 							if ( isRelative(resolvedId) || isAbsolute(resolvedId) ) {
 								// Try to deduce relative path from entry dir if resolvedId is defined as a relative path.

test/function/external-function/_config.js

@@ -0,0 +1,14 @@
+module.exports = {
+	description: 'allows external option to be a function (#522)',
+	options: {
+		external: id => {
+			return id === 'external';
+		}
+	},
+	context: {
+		require: id => {
+			if ( id === 'external' ) return 42;
+			return require( id );
+		}
+	}
+};

test/function/external-function/main.js

@@ -0,0 +1,2 @@
+import ext from 'external';
+assert.equal( ext, 42 );