Issue #146 ExpiringValue caches failures

- New default behavior: If the factory function rejects/throws, the value is returned but immediately expired. The next call to `get()` will retry.
- New option on the constructor to choose to cache errors instead - the old behavior.
- Major version bump because of default behavior change.

docs/types/expiring-value.d.ts

@@ -5,6 +5,7 @@
 export declare class ExpiringValue<T> {
     private factoryFn;
     private ttl;
+    private options;
     /** Cached value */
     private value;
     /** Epoch millisecond time of when the current value expires */
@@ -14,8 +15,13 @@ export declare class ExpiringValue<T> {
      *
      * @param factoryFn factory to lazy-load the value
      * @param ttl milliseconds the value is good for, after which it is reloaded.
+     * @param options optional options to change behavior
+     * @param options.cacheError set to true to cache for TTL a Promise rejection from factoryFn.
+     *        By default, a rejection is not cached and factoryFn will be retried upon the next call.
      */
-    constructor(factoryFn: (() => Promise<T>), ttl: number);
+    constructor(factoryFn: (() => Promise<T>), ttl: number, options?: {
+        cacheError: boolean;
+    });
     /**
      * Get value; lazy-load from factory if not yet loaded or if expired.
      */
@@ -29,4 +35,6 @@ export declare class ExpiringValue<T> {
      * Is the value expired (or not set)
      */
     isExpired(): boolean;
+    /** Reset the value expiration to TTL past now */
+    private extendExpiration;
 }

expiring-value/lib/expiring-value.test.ts

@@ -1,8 +1,8 @@
-import {ExpiringValue} from "./expiring-value";
 import * as MockDate from "mockdate";
+import {ExpiringValue} from "./expiring-value";
 
 
-describe('ExpiringValue',() => {
+describe('ExpiringValue', () => {
     const baseDate = Date.now();
 
     beforeEach(() => {
@@ -18,7 +18,7 @@ describe('ExpiringValue',() => {
 
         // Initialize
         let sut = new ExpiringValue<string>(() => Promise.resolve(factoryValue), 1000);
-        expect(sut['value']).toBeFalsy();
+        expect(sut['value']).toBeUndefined();
         expect(sut['expiration']).toEqual(0);
 
         // First GET - lazy created
@@ -52,7 +52,7 @@ describe('ExpiringValue',() => {
 
         // Clear content..
         sut.clear();
-        expect(sut['value']).toBeFalsy();
+        expect(sut['value']).toBeUndefined();
         expect(sut['expiration']).toEqual(0);
 
         // Fourth GET - factory called again
@@ -64,4 +64,41 @@ describe('ExpiringValue',() => {
         await expect(sut['value']).resolves.toBe('world!');
         expect(sut['expiration']).toEqual(baseDate + 1000);
     });
+
+    test("doesn't cache failure", async () => {
+        let factoryResponse: Promise<string> = Promise.reject(new Error());
+
+        // Initialize
+        let sut = new ExpiringValue<string>(() => factoryResponse, 1000);
+        expect(sut['value']).toBeUndefined();
+        expect(sut['expiration']).toEqual(0);
+
+        // First GET - rejects - still expired
+        await expect(sut.get()).rejects.toThrow();
+        expect(sut['expiration']).toEqual(0);
+
+        // Second GET - calls again, success this time
+        factoryResponse = Promise.resolve("yay");
+        const v2 = await sut.get();
+        expect(v2).toBe('yay');
+        await expect(sut['value']).resolves.toBe('yay');
+        expect(sut['expiration']).toEqual(baseDate + 1000);
+    });
+
+    test("does cache failure when option selected", async () => {
+        let factoryResponse: Promise<string> = Promise.reject(new Error());
+
+        // Initialize
+        let sut = new ExpiringValue<string>(() => factoryResponse, 1000,{cacheError: true});
+        expect(sut['value']).toBeUndefined();
+        expect(sut['expiration']).toEqual(0);
+
+        // First GET - rejects - still expired
+        await expect(sut.get()).rejects.toThrow();
+        expect(sut['expiration']).toEqual(baseDate + 1000);
+
+        // Second GET - calls again, uses cached failure
+        factoryResponse = Promise.resolve("yay");
+        await expect(sut.get()).rejects.toThrow();
+    });
 });

expiring-value/lib/expiring-value.ts

@@ -4,7 +4,7 @@
  */
 export class ExpiringValue<T> {
     /** Cached value */
-    private value: Promise<T>|undefined;
+    private value: Promise<T> | undefined;
 
     /** Epoch millisecond time of when the current value expires */
     private expiration: number = 0;
@@ -14,8 +14,15 @@ export class ExpiringValue<T> {
      *
      * @param factoryFn factory to lazy-load the value
      * @param ttl milliseconds the value is good for, after which it is reloaded.
+     * @param options optional options to change behavior
+     * @param options.cacheError set to true to cache for TTL a Promise rejection from factoryFn.
+     *        By default, a rejection is not cached and factoryFn will be retried upon the next call.
      */
-    constructor(private factoryFn: (() => Promise<T>), private ttl: number) {
+    constructor(
+        private factoryFn: (() => Promise<T>),
+        private ttl: number,
+        private options = {cacheError: false}
+    ) {
     }
 
     /**
@@ -24,7 +31,13 @@ export class ExpiringValue<T> {
     get(): Promise<T> {
         if (this.isExpired()) {
             this.value = this.factoryFn();
-            this.expiration = Date.now() + this.ttl;
+
+            if (this.options.cacheError) {
+                this.extendExpiration();
+            } else {
+                // Update expiration, only upon success
+                this.value.then(() => this.extendExpiration());
+            }
         }
 
         return this.value!;
@@ -45,4 +58,9 @@ export class ExpiringValue<T> {
     isExpired(): boolean {
         return Date.now() > this.expiration;
     }
+
+    /** Reset the value expiration to TTL past now */
+    private extendExpiration(): void {
+        this.expiration = Date.now() + this.ttl;
+    }
 }