merge-partially

merge-partially

mergePartially is a convenience method for overwriting only the values you want

I see lots of TypeScript stuff. Can I use this in JavaScript too?

Yes. Even though the examples are in TypeScript (since it helps to illustrate the problem that mergePartially solves), you can just remove the type annotations when using mergePartially.

Why would you want to use this:

tl;dr: with mergePartially helps you fold objects together without overwriting the originals. You can have less brittle tests but with all the flexibility you need.

There are many use cases, but I find this function to be most useful in testing scenarios.

Context: Often times when creating a factory function for tests, you want to be able to create a function that

interface IUser {
    id: number;
    firstName: string;
    lastName: string;
    age: number;
}

function makeFakeUser(): IUser {
    return {
        id: 1,
        age: 42,
        firstName: "John",
        lastName: "Smith"
    }
}

But what happens when unit test #2 needs the firstName value to be different? If you change the hard-coded value inside of makeFakeUser, then you break unit test #1. So if you don't proceed carefully, then makeFakeUser is at risk of creating brittle tests!

A more flexible approach is provide default values and allow the user to provide their own values.

First, let's try to write the flexible factory function without mergePartially

Ugh this is gonna be long...

function makeFakeUser(overrides?: Partial<IUser>): IUser {
    const defaults = {
        id: 1,
        age: 42,
        firstName: "John",
        lastName: "Smith"
    };

    const result = {
        id: overrides && overrides.id !== undefined ? overrides.id : defaults.id,
        age: overrides && overrides.age !== undefined ? overrides.age : defaults.age,
        firstName: overrides && overrides.firstName !== undefined ? overrides.firstName : defaults.firstName,
        lastName: overrides && overrides.lastName !== undefined ? overrides.lastName : defaults.lastName,
    }

    return result;
}

Now let's refactor using mergePartially

Wow look how much fewer lines and characters we have to write to accomplish the same thing:

function makeFakeUser(overrides?: Partial<IUser>): IUser {
    return mergePartially({
        id: 1,
        age: 42,
        firstName: "John",
        lastName: "Smith"
    }, overrides);
}

Examples

See the unit tests for various examples