Adds decrement() to ParseObject

src/ParseObject.js

@@ -764,6 +764,24 @@ class ParseObject {
     return this.set(attr, new IncrementOp(amount));
   }
 
+  /**
+   * Atomically decrements the value of the given attribute the next time the
+   * object is saved. If no amount is specified, 1 is used by default.
+   *
+   * @param attr {String} The key.
+   * @param amount {Number} The amount to decrement by (optional).
+   * @return {(ParseObject|Boolean)}
+   */
+  decrement(attr: string, amount?: number): ParseObject | boolean {
+    if (typeof amount === 'undefined') {
+      amount = 1;
+    }
+    if (typeof amount !== 'number') {
+      throw new Error('Cannot decrement by a non-numeric amount.');
+    }
+    return this.set(attr, new IncrementOp(amount * -1));
+  }
+
   /**
    * Atomically add an object to the end of the array associated with a given
    * key.

src/__tests__/ParseObject-test.js

@@ -491,6 +491,50 @@ describe('ParseObject', () => {
     expect(o2.attributes).toEqual({ age: 41 });
   });
 
+
+  it('can decrement a field', () => {
+    const o = new ParseObject('Person');
+    o.decrement('age');
+    expect(o.attributes).toEqual({ age: -1 });
+    expect(o.op('age') instanceof IncrementOp).toBe(true);
+    expect(o.dirtyKeys()).toEqual(['age']);
+    expect(o._getSaveJSON()).toEqual({
+      age: { __op: 'Increment', amount: -1 }
+    });
+
+    o.decrement('age', 4);
+    expect(o.attributes).toEqual({ age: -5 });
+    expect(o._getSaveJSON()).toEqual({
+      age: { __op: 'Increment', amount: -5 }
+    });
+
+    expect(o.decrement.bind(o, 'age', 'four')).toThrow(
+      'Cannot decrement by a non-numeric amount.'
+    );
+    expect(o.decrement.bind(o, 'age', null)).toThrow(
+      'Cannot decrement by a non-numeric amount.'
+    );
+    expect(o.decrement.bind(o, 'age', { amount: 4 })).toThrow(
+      'Cannot decrement by a non-numeric amount.'
+    );
+
+    o.set('age', 30);
+    o.decrement('age');
+    expect(o.attributes).toEqual({ age: 29 });
+    expect(o._getSaveJSON()).toEqual({
+      age: 29
+    });
+
+    const o2 = new ParseObject('Person');
+    o2._finishFetch({
+      objectId: 'ABC123',
+      age: 40
+    });
+    expect(o2.attributes).toEqual({ age: 40 });
+    o2.decrement('age');
+    expect(o2.attributes).toEqual({ age: 39 });
+  });
+
   it('can set nested field', () => {
     const o = new ParseObject('Person');
     o._finishFetch({