default to array when update keys are integers

lib/update.js

@@ -1,8 +1,6 @@
 import wrap from './wrap';
-
-function isEmpty(object) {
-  return !Object.keys(object).length;
-}
+import isEmpty from './util/isEmpty';
+import defaultObject from './util/defaultObject';
 
 function reduce(object, callback, initialValue) {
   return Object.keys(object).reduce((acc, key) => {
@@ -64,9 +62,7 @@ function update(updates, object, ...args) {
     return updates;
   }
 
-  const defaultedObject = (typeof object === 'undefined' || object === null) ?
-    {} :
-    object;
+  const defaultedObject = defaultObject(object, updates);
 
   const resolvedUpdates = resolveUpdates(updates, defaultedObject);
 

lib/util/defaultObject.js

@@ -0,0 +1,35 @@
+import isEmpty from './isEmpty';
+
+function isInt(value) {
+  if (isNaN(value)) {
+    return false;
+  }
+  const x = parseFloat(value);
+  return (x | 0) === x;
+}
+
+function isArrayUpdate(updates) {
+  for (const updateKey of Object.keys(updates)) {
+    if (!isInt(updateKey)) { return false; }
+  }
+
+  return true;
+}
+
+function arrayOrObject(updates) {
+  if (!isEmpty(updates) && isArrayUpdate(updates)) {
+    return [];
+  }
+
+  return {};
+}
+
+function defaultObject(object, updates) {
+  if (typeof object === 'undefined' || object === null) {
+    return arrayOrObject(updates);
+  }
+
+  return object;
+}
+
+export default defaultObject;

lib/util/isEmpty.js

@@ -0,0 +1,5 @@
+function isEmpty(object) {
+  return !Object.keys(object).length;
+}
+
+export default isEmpty;

test/updateIn-spec.js

@@ -27,6 +27,12 @@ describe('u.updateIn', () => {
     expect(result).to.eql({ a: [0, 3, 0] });
   });
 
+  it('can create array if all keys are numbers', () => {
+    const result = u.updateIn('a.1', 3, null);
+
+    expect(result).to.eql({ a: [, 3] }); // eslint-disable-line no-sparse-arrays
+  });
+
   it('can be partially applied', () => {
     const object = { a: { b: 0 } };
     const result = u.updateIn('a.b')(3)(object);

test/updeep-spec.js

@@ -42,6 +42,18 @@ describe('updeep', () => {
     expect(u(null, {})).to.be.null;
   });
 
+  it('can create array if all keys are numbers', () => {
+    const result = u({ 0: 'hi', '1': 'world' }, null);
+
+    expect(result).to.eql(['hi', 'world']);
+  });
+
+  it('does not create array if any key is not number', () => {
+    const result = u({ 0: 'hi', '1a': 'world' }, null);
+
+    expect(result).to.eql({ 0: 'hi', '1a': 'world' });
+  });
+
   it('can add an element to an array', () => {
     const object = [];
     const result = u({ 0: 3 }, object);