fix(single): predicate function receives indicies starting at 0 (#2396)

If the user is relying on indicies starting at 1 than this could be a BC. Otherwise now it's

compatible with RxJS 4.

Author: martinsik

spec/operators/single-spec.ts

@@ -1,3 +1,4 @@
+import {expect} from 'chai';
 import * as Rx from '../../dist/cjs/Rx';
 import marbleTestingSignature = require('../helpers/marble-testing'); // tslint:disable-line:no-require-imports
 
@@ -151,4 +152,21 @@ describe('Observable.prototype.single', () => {
     expectObservable(e1.single(predicate)).toBe(expected, {z: undefined});
     expectSubscriptions(e1.subscriptions).toBe(e1subs);
   });
+
+  it('should call predicate with indices starting at 0', () => {
+    const e1 =    hot('--a--b--c--|');
+    const e1subs =    '^          !';
+    const expected =  '-----------(b|)';
+
+    let indices = [];
+    const predicate = function(value, index) {
+      indices.push(index);
+      return value === 'b';
+    };
+
+    expectObservable(e1.single(predicate).do(null, null, () => {
+      expect(indices).to.deep.equal([0, 1, 2]);
+    })).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
 });

src/operator/single.ts

@@ -61,19 +61,18 @@ class SingleSubscriber<T> extends Subscriber<T> {
   }
 
   protected _next(value: T): void {
-    const predicate = this.predicate;
-    this.index++;
-    if (predicate) {
-      this.tryNext(value);
+    const index = this.index++;
+
+    if (this.predicate) {
+      this.tryNext(value, index);
     } else {
       this.applySingleValue(value);
     }
   }
 
-  private tryNext(value: T): void {
+  private tryNext(value: T, index: number): void {
     try {
-      const result = this.predicate(value, this.index, this.source);
-      if (result) {
+      if (this.predicate(value, index, this.source)) {
         this.applySingleValue(value);
       }
     } catch (err) {