feat(assertions): capture matching value

packages/@aws-cdk/assertions/README.md

@@ -184,7 +184,9 @@ assert.hasResourceProperties('Foo::Bar', {
 The `Match.objectEquals()` API can be used to assert a target as a deep exact
 match.
 
-In addition, the `Match.absentProperty()` can be used to specify that a specific
+### Presence and Absence
+
+The `Match.absentProperty()` matcher can be used to specify that a specific
 property should not exist on the target. This can be used within `Match.objectLike()`
 or outside of any matchers.
 
@@ -218,6 +220,42 @@ assert.hasResourceProperties('Foo::Bar', {
 });
 ```
 
+The `Match.anyValue()` matcher can be used to specify that a specific value should be found
+at the location. This matcher will fail if when the target location has null-ish values
+(i.e., `null` or `undefined`).
+
+This matcher can be combined with any of the other matchers.
+
+```ts
+// Given a template -
+// {
+//   "Resources": {
+//     "MyBar": {
+//       "Type": "Foo::Bar",
+//       "Properties": {
+//         "Fred": {
+//           "Wobble": ["Flob", "Flib"],
+//         }
+//       }
+//     }
+//   }
+// }
+
+// The following will NOT throw an assertion error
+assert.hasResourceProperties('Foo::Bar', {
+  Fred: {
+    Wobble: [Match.anyValue(), "Flip"],
+  },
+});
+
+// The following will throw an assertion error
+assert.hasResourceProperties('Foo::Bar', {
+  Fred: {
+    Wimble: Match.anyValue(),
+  },
+});
+```
+
 ### Array Matchers
 
 The `Match.arrayWith()` API can be used to assert that the target is equal to or a subset
@@ -283,6 +321,37 @@ assert.hasResourceProperties('Foo::Bar', Match.objectLike({
 }});
 ```
 
+## Capturing Values
+
+This matcher APIs documented above allow capturing values in the matching entry
+(Resource, Output, Mapping, etc.). The following code captures a string from a
+matching resource.
+
+```ts
+// Given a template -
+// {
+//   "Resources": {
+//     "MyBar": {
+//       "Type": "Foo::Bar",
+//       "Properties": {
+//         "Fred": ["Flob", "Cat"],
+//         "Waldo": ["Qix", "Qux"],
+//       }
+//     }
+//   }
+// }
+
+const fredCapture = new Capture();
+const waldoCapture = new Capture();
+assert.hasResourceProperties('Foo::Bar', {
+  Fred: fredCapture,
+  Waldo: ["Qix", waldoCapture],
+});
+
+fredCapture.asArray(); // returns ["Flob", "Cat"]
+waldoCapture.asString(); // returns "Qux"
+```
+
 ## Strongly typed languages
 
 Some of the APIs documented above, such as `templateMatches()` and

packages/@aws-cdk/assertions/lib/capture.ts

@@ -0,0 +1,98 @@
+import { Matcher, MatchResult } from './matcher';
+import { Type, getType } from './private/type';
+
+/**
+ * Capture values while matching templates.
+ * Using an instance of this class within a Matcher will capture the matching value.
+ * The `as*()` APIs on the instance can be used to get the captured value.
+ */
+export class Capture extends Matcher {
+  public readonly name: string;
+  private value: any = null;
+
+  constructor() {
+    super();
+    this.name = 'Capture';
+  }
+
+  public test(actual: any): MatchResult {
+    this.value = actual;
+
+    const result = new MatchResult(actual);
+    if (actual == null) {
+      result.push(this, [], `Can only capture non-nullish values. Found ${actual}`);
+    }
+    return result;
+  }
+
+  /**
+   * Retrieve the captured value as a string.
+   * An error is generated if no value is captured or if the value is not a string.
+   */
+  public asString(): string {
+    this.checkNotNull();
+    if (getType(this.value) === 'string') {
+      return this.value;
+    }
+    this.reportIncorrectType('string');
+  }
+
+  /**
+   * Retrieve the captured value as a number.
+   * An error is generated if no value is captured or if the value is not a number.
+   */
+  public asNumber(): number {
+    this.checkNotNull();
+    if (getType(this.value) === 'number') {
+      return this.value;
+    }
+    this.reportIncorrectType('number');
+  }
+
+  /**
+   * Retrieve the captured value as a boolean.
+   * An error is generated if no value is captured or if the value is not a boolean.
+   */
+  public asBoolean(): boolean {
+    this.checkNotNull();
+    if (getType(this.value) === 'boolean') {
+      return this.value;
+    }
+    this.reportIncorrectType('boolean');
+  }
+
+  /**
+   * Retrieve the captured value as an array.
+   * An error is generated if no value is captured or if the value is not an array.
+   */
+  public asArray(): any[] {
+    this.checkNotNull();
+    if (getType(this.value) === 'array') {
+      return this.value;
+    }
+    this.reportIncorrectType('array');
+  }
+
+  /**
+   * Retrieve the captured value as a JSON object.
+   * An error is generated if no value is captured or if the value is not an object.
+   */
+  public asObject(): { [key: string]: any } {
+    this.checkNotNull();
+    if (getType(this.value) === 'object') {
+      return this.value;
+    }
+    this.reportIncorrectType('object');
+  }
+
+  private checkNotNull(): void {
+    if (this.value == null) {
+      throw new Error('No value captured');
+    }
+  }
+
+  private reportIncorrectType(expected: Type): never {
+    throw new Error(`Captured value is expected to be ${expected} but found ${getType(this.value)}. ` +
+      `Value is ${JSON.stringify(this.value, undefined, 2)}`);
+  }
+}

packages/@aws-cdk/assertions/lib/index.ts

@@ -1,3 +1,4 @@
+export * from './capture';
 export * from './template';
 export * from './match';
 export * from './matcher';

packages/@aws-cdk/assertions/lib/match.ts

@@ -1,4 +1,5 @@
 import { Matcher, MatchResult } from './matcher';
+import { getType } from './private/type';
 import { ABSENT } from './vendored/assert';
 
 /**
@@ -63,6 +64,13 @@ export abstract class Match {
   public static not(pattern: any): Matcher {
     return new NotMatch('not', pattern);
   }
+
+  /**
+   * Matches any non-null value at the target.
+   */
+  public static anyValue(): Matcher {
+    return new AnyMatch('anyValue');
+  }
 }
 
 /**
@@ -141,22 +149,22 @@ interface ArrayMatchOptions {
  * Match class that matches arrays.
  */
 class ArrayMatch extends Matcher {
-  private readonly partial: boolean;
+  private readonly subsequence: boolean;
 
   constructor(
     public readonly name: string,
     private readonly pattern: any[],
     options: ArrayMatchOptions = {}) {
 
     super();
-    this.partial = options.subsequence ?? true;
+    this.subsequence = options.subsequence ?? true;
   }
 
   public test(actual: any): MatchResult {
     if (!Array.isArray(actual)) {
       return new MatchResult(actual).push(this, [], `Expected type array but received ${getType(actual)}`);
     }
-    if (!this.partial && this.pattern.length !== actual.length) {
+    if (!this.subsequence && this.pattern.length !== actual.length) {
       return new MatchResult(actual).push(this, [], `Expected array of length ${this.pattern.length} but received ${actual.length}`);
     }
 
@@ -166,10 +174,16 @@ class ArrayMatch extends Matcher {
     const result = new MatchResult(actual);
     while (patternIdx < this.pattern.length && actualIdx < actual.length) {
       const patternElement = this.pattern[patternIdx];
+
       const matcher = Matcher.isMatcher(patternElement) ? patternElement : new LiteralMatch(this.name, patternElement);
+      if (this.subsequence && matcher instanceof AnyMatch) {
+        // array subsequence matcher is not compatible with anyValue() matcher. They don't make sense to be used together.
+        throw new Error('The Matcher anyValue() cannot be nested within arrayWith()');
+      }
+
       const innerResult = matcher.test(actual[actualIdx]);
 
-      if (!this.partial || !innerResult.hasFailed()) {
+      if (!this.subsequence || !innerResult.hasFailed()) {
         result.compose(`[${actualIdx}]`, innerResult);
         patternIdx++;
         actualIdx++;
@@ -271,6 +285,16 @@ class NotMatch extends Matcher {
   }
 }
 
-function getType(obj: any): string {
-  return Array.isArray(obj) ? 'array' : typeof obj;
+class AnyMatch extends Matcher {
+  constructor(public readonly name: string) {
+    super();
+  }
+
+  public test(actual: any): MatchResult {
+    const result = new MatchResult(actual);
+    if (actual == null) {
+      result.push(this, [], 'Expected a value but found none');
+    }
+    return result;
+  }
 }

packages/@aws-cdk/assertions/lib/matcher.ts

@@ -17,6 +17,8 @@ export abstract class Matcher {
 
   /**
    * Test whether a target matches the provided pattern.
+   * Every Matcher must implement this method.
+   * This method will be invoked by the assertions framework. Do not call this method directly.
    * @param actual the target to match
    * @return the list of match failures. An empty array denotes a successful match.
    */

packages/@aws-cdk/assertions/lib/private/type.ts

@@ -0,0 +1,5 @@
+export type Type = 'string' | 'number' | 'bigint' | 'boolean' | 'symbol' | 'undefined' | 'object' | 'function' | 'array';
+
+export function getType(obj: any): Type {
+  return Array.isArray(obj) ? 'array' : typeof obj;
+}

packages/@aws-cdk/assertions/test/capture.test.ts

@@ -0,0 +1,69 @@
+import { Capture, Match } from '../lib';
+
+describe('Capture', () => {
+  test('uncaptured', () => {
+    const capture = new Capture();
+    expect(() => capture.asString()).toThrow(/No value captured/);
+  });
+
+  test('nullish', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: capture });
+
+    const result = matcher.test({ foo: null });
+    expect(result.failCount).toEqual(1);
+    expect(result.toHumanStrings()[0]).toMatch(/Can only capture non-nullish values/);
+  });
+
+  test('asString()', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: capture });
+
+    matcher.test({ foo: 'bar' });
+    expect(capture.asString()).toEqual('bar');
+
+    matcher.test({ foo: 3 });
+    expect(() => capture.asString()).toThrow(/expected to be string but found number/);
+  });
+
+  test('asNumber()', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: capture });
+
+    matcher.test({ foo: 3 });
+    expect(capture.asNumber()).toEqual(3);
+
+    matcher.test({ foo: 'bar' });
+    expect(() => capture.asNumber()).toThrow(/expected to be number but found string/);
+  });
+
+  test('asArray()', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: capture });
+
+    matcher.test({ foo: ['bar'] });
+    expect(capture.asArray()).toEqual(['bar']);
+
+    matcher.test({ foo: 'bar' });
+    expect(() => capture.asArray()).toThrow(/expected to be array but found string/);
+  });
+
+  test('asObject()', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: capture });
+
+    matcher.test({ foo: { fred: 'waldo' } });
+    expect(capture.asObject()).toEqual({ fred: 'waldo' });
+
+    matcher.test({ foo: 'bar' });
+    expect(() => capture.asObject()).toThrow(/expected to be object but found string/);
+  });
+
+  test('nested within an array', () => {
+    const capture = new Capture();
+    const matcher = Match.objectEquals({ foo: ['bar', capture] });
+
+    matcher.test({ foo: ['bar', 'baz'] });
+    expect(capture.asString()).toEqual('baz');
+  });
+});