chore: low-hanging bundle size improvements in apollo-link (#970)

packages/apollo-link/src/link.ts

@@ -17,27 +17,30 @@ import {
   createOperation,
 } from './linkUtils';
 
-const passthrough = (op, forward) => (forward ? forward(op) : Observable.of());
+function passthrough(op, forward) {
+  return forward ? forward(op) : Observable.of();
+}
 
-const toLink = (handler: RequestHandler | ApolloLink) =>
-  typeof handler === 'function' ? new ApolloLink(handler) : handler;
+function toLink(handler: RequestHandler | ApolloLink) {
+  return typeof handler === 'function' ? new ApolloLink(handler) : handler;
+}
 
-export const empty = (): ApolloLink =>
-  new ApolloLink((op, forward) => Observable.of());
+export function empty(): ApolloLink {
+  return new ApolloLink(() => Observable.of());
+}
 
-export const from = (links: ApolloLink[]): ApolloLink => {
+export function from(links: ApolloLink[]): ApolloLink {
   if (links.length === 0) return empty();
-
   return links.map(toLink).reduce((x, y) => x.concat(y));
-};
+}
 
-export const split = (
+export function split(
   test: (op: Operation) => boolean,
   left: ApolloLink | RequestHandler,
-  right: ApolloLink | RequestHandler = new ApolloLink(passthrough),
-): ApolloLink => {
+  right?: ApolloLink | RequestHandler,
+): ApolloLink {
   const leftLink = toLink(left);
-  const rightLink = toLink(right);
+  const rightLink = toLink(right || new ApolloLink(passthrough));
 
   if (isTerminating(leftLink) && isTerminating(rightLink)) {
     return new ApolloLink(operation => {
@@ -52,7 +55,7 @@ export const split = (
         : rightLink.request(operation, forward) || Observable.of();
     });
   }
-};
+}
 
 // join two Links together
 export const concat = (
@@ -103,9 +106,9 @@ export class ApolloLink {
   public split(
     test: (op: Operation) => boolean,
     left: ApolloLink | RequestHandler,
-    right: ApolloLink | RequestHandler = new ApolloLink(passthrough),
+    right?: ApolloLink | RequestHandler,
   ): ApolloLink {
-    return this.concat(split(test, left, right));
+    return this.concat(split(test, left, right || new ApolloLink(passthrough)));
   }
 
   public concat(next: ApolloLink | RequestHandler): ApolloLink {