feat(watch): watch setters on an object

src/IWatchHandler.ts

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+export interface IWatchHandler {
+  (value?: any): void;
+}

src/watch.spec.ts

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+import { watch } from './watch';
+
+describe('watch', () => {
+  const prop = 'PROPERTY';
+  const value = 'VALUE';
+  let o;
+
+  beforeEach(() => {
+    o = {};
+  });
+
+  it('watches for change', () => {
+    let counter = 0;
+    watch(o, prop, () => {
+      counter++;
+    });
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(2);
+  });
+
+  it('watches for change once', () => {
+    let counter = 0;
+    watch(
+      o,
+      prop,
+      () => {
+        counter++;
+      },
+      true
+    );
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(1);
+  });
+
+  it('unwatches and keeps set value', () => {
+    const unwatch = watch(o, prop, () => {});
+
+    o[prop] = value;
+    unwatch();
+
+    expect(o[prop]).toBe(value);
+  });
+
+  it('can nest watches', () => {
+    let counter = 0;
+    watch(o, prop, () => {
+      counter++;
+    });
+    watch(o, prop, () => {
+      counter++;
+    });
+
+    o[prop] = value;
+    o[prop] = value;
+
+    expect(counter).toBe(4);
+  });
+});

src/watch.ts

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+import { IWatchHandler } from './IWatchHandler';
+
+export function watch(
+  o: any,
+  property: string,
+  handler: IWatchHandler,
+  once = false
+): () => void {
+
+  const descriptor = Object.getOwnPropertyDescriptor(o, property) || {};
+
+  let getValue = descriptor.get || (() => descriptor.value);
+  let setValue = descriptor.set || (value => (descriptor.value = value));
+
+  const unwatch = () => {
+    delete o[property];
+    o[property] = getValue();
+  };
+
+  Object.defineProperty(o, property, {
+    get: getValue,
+    set(value) {
+      if (once && getValue() === undefined) {
+        setValue(value);
+        unwatch();
+        handler(value);
+
+        return;
+      }
+
+      setValue(value);
+      handler(value);
+    },
+    configurable: true
+  });
+
+  return unwatch;
+}