Utility function for merging an object into a reactive object in Vue

This library was designed to efficiently and automatically deep-merge objects with Vue-managed objects.

I really enjoy Vue and Vuex's philosophy -- update your state, and your UI will reactively update automatically. But what I don't enjoy is writing code to wire API JSON responses back into my Vue data or Vuex state objects (i.e., mutation functions):

state: {
	customer {
		firstName: "",
		lastName: ""
		// ...
	}
},
actions: {
	getCustomer: id => axios
		.get("/api/getCustomer/" + id)
		.then(function(response) {
			context.commit("COMMIT_CUSTOMER", response.data)
		})
},
mutations: {
	COMMIT_CUSTOMER(state, data) {
		state.customer.firstName = data.firstName;
		state.customer.lastName = data.lastName;
		// ... sigh ...
	},
}

(The examples here are specific to Vuex, but apply equally if you're just using the data object to store your state.)

If an object is replaced entirely, all you have to do is replace one object with another. But if the API needs to return partial object changes (perhaps even changes across several portions of the page state, such as updating a table and a page header and three menu options), you have to manually wire the bits and pieces. Worse, if each API call can return different portions of state, you'll be writing a LOT of boilerplate code.

Vue-Object-Merge provides a single function, stateMerge, that performs a deep merge of one object into another. Basically, it greatly simplifies mapping keys from object A into Vue object B, without disturbing keys in B that are not included in A:

mutations: {
	// I can call this single mutation for any API response that sparely
	// matches anything in the Vuex state.
	MERGE_STATE: (state, response) => stateMerge(state, response),
}

Now, many of my Vuex actions call their API endpoint and all commit a single mutation (as above). This MERGE_STATE mutation folds the keys and values returned from the API into the application state in one step, and it's all done in a way that Vue can react properly to the changes (i.e., it uses Vue.set()).

The API need not return a full representation of the state. For example, if an API call needs to update state.ui.userForm.fields, the API can just return the contents of the fields object (or any portion thereof) and your action can look like this:

context.commit("MERGE_STATE", { ui: { userForm: { fields: response.data } } })

This will navigate stateMerge directly to the place in your state where the response data should be merged, leaving the rest of your state unaffected (and even any keys of ui.userForm.fields that aren't part of the JSON response).

In addition to API calls, it can also be used for, say, updating data elements that are two-way-bound to form fields to set them to a Vuex state object, or vice versa to merge changes from the form back into the Vuex state.

stateMerge(state, value, propName, ignoreNull)

where:

• state is the object to be updated.
• value is the object or value containing the change(s).
• propName is the key of state to be modified (required if value isn't an object, otherwise optional). This was primarily intended for internal use for recursion, but can be handy in other situations.
• ignoreNull should be set to true (default is false) if a null value should not overwrite state's value.

Given objects state and value, stateMerge will:

• Traverse the attributes of value
• If the attribute is a primitive (number, boolean, etc.) or built-in object (array, date, regex, etc.), it will overwrite state's attribute with the new value (adding the attribute if it doesn't exist).
• If the attribute is a custom object (normal JavaScript associative array), it will recurse into that object's attributes with the same logic.
• If the attribute is null, it will decide what to do based on the ignoreNull argument.

The ignoreNull option was added to make it easier to use the same server-side object (.NET "POCOs" in my case) to service different requests, where portions of the response object not modified are set to null. This has a small data overhead over bespoke responses for each API endpoint, but encourages code reuse and consistency.

Technical implementation note: testing the "type" of a value can be complicated--typeof, instanceof, and other methods all have pros and cons. In this case, there was a simple answer: Object.prototype.toString.call(value). This returns the string [object Object] for all user-defined objects (the ones we want to recurse into), and returns various other values for built-in JavaScript types like Date, Array, RegEx, Number, String, Boolean, Math, Function, null, and undefined--the keys we usually want to overwrite.

• This only works if your value object forms a directed acyclic graph, otherwise you'll have an endless loop when updating.
• This overwrites arrays, it does not merge them. It only merges objects.

Here's a CodePen where you can play with merging an object into a sample Vuex state: https://codepen.io/richardtallent/pen/eyWKGN

Basic example:

var a = { foo: 1, bar: 0 }
var b = { foo: 2, fizz: 4, fee: null }
stateMerge(a, b)
console.log(a)
// { foo: 2, bar: 0, fizz: 4, fee: null }

With the ignoreNull option:

var a = { foo: 1, fee: { id: 1 } }
var b = { foo: 2, fee: null }
stateMerge(a, b, null, true)
console.log(a)
// { foo: 2, fee: { id: 1 } }

Example of a deeper merge:

var a = { foo: [0, 1], bar: { "1": "Marcia", "2": "Peter" } }
var b = { foo: [2, 3], bar: { "1": "Jan" } }
stateMerge(a, b)
console.log(a)
// { foo: [2, 3], bar: { "1" : "Jan", "2": "Peter" } }

Using the optional propName parameter:

var a = { foo: [0, 1], bar: { "1": "Marcia", "2": "Peter" } }
var b = { "1": "Jan" }
stateMerge(a, b, "bar")
console.log(a)
// { foo: [0, 1], bar: { "1" : "Jan", "2": "Peter" } }

import { stateMerge } from "vue-object-merge"

export const store = new Vuex.Store({
...
  actions: {
	getOrders(context)) {
		return HTTP.get("/orders")
			.then(function(response)) {
				context.commit("MERGE_STATE", response.data)
			})
	}
  },
  mutations: {
	MERGE_STATE(state, data) {
		stateMerge(state, data)
	}
}