Nick Raphael

Posted on Nov 25, 2019

Typescript Partial<T>, where have you been my whole life?

#angular #typescript

Oh boy, how did I not know about Partial until now? This is something all Typescripters need to know.

I'll use the example based on the official docs...
https://www.typescriptlang.org/docs/handbook/utility-types.html

Let's sasy you have a simple interface and instance...

interface Todo {
    title: string;
    description: string;
}

const todo1 = {
    title: 'organize desk',
    description: 'clear clutter',
};

How would we write a method that takes our instance of Todo and updates it with values from another Todo?

How about this?

function updateTodo(originalTodo: Todo, fieldsToUpdateTodo: any) {
    return { ...originalTodo, ...fieldsToUpdateTodo };
}

const todo2 = updateTodo(todo1, {
    description: 'throw out trash',
});

Not great. We had to type fieldsToUpdateTodo as any because if it was an Todo, we would need to set every property on the Todo interface. What if we only wanted to update description? Hence using any. I guess we could mark all properties in Todo as optional, but then we'd lose a lot of the typechecking that we love.

If we knew we only ever wanted to update description things are easier...

function updateTodoDescription(originalTodo: ITodo, description: string) {
    return { ...originalTodo, description: description };
}

But this pattern isn't scaleable if we had many properties and wanted to arbitrarily update properties.

Let's cut to the chase. How can Partial help? Turns out it's simple, barely an inconvenience...

function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
    return { ...todo, ...fieldsToUpdate }; 
}

Wrapping an object in Partial marks all the properties on that object as optional.

We can then call our updateTodo like this...

const todo2 = updateTodo(todo1, {
    description: 'throw out trash',
});

We are not forced to set every property from the Todo interface. Our UpdateTodo method can then happily use the spread operator to merge the two Todo's.

Top comments (19)

emptyother • Nov 26 '19

I normally use Partial<T> interface when making class constructors.

class MyClass {
    myProp = 0; // Default values are applied first.
    constructor(cfg: Partial<MyClass> = {}) {
        extend(this, cfg); // Method that shallow-copies properties from cfg to this, overwriting any defaults.
    }
}

Paul Cook • Dec 3 '19

I do the same except with Object.assign with partials like so:

export class User {
    id: number;
    name: string;
    profile = new Profile();

    constructor(options?: Partial<User>) {
        Object.assign(this, options);
    }
}

Works really nicely for creating and updating data.

onChangeUser(event: FormChangeEvent<UserForm>): void {
    this.user = new UpsertUser({
        ...this.user,
        ...event.value,
        profile: new Profile({
            ...this.user.profile,
            ...event.value.profile
        })
    });
}

Rather than having a web of Object.assign everywhere you actually see and work with the shape of your data.

Daniel Lo Nigro • Nov 26 '19

Your extend method just looks like Object.assign :)

emptyother • Nov 27 '19 • Edited

Completely forgot that one exists. Symptom of working too much with an old IE11-supported javascript framework. :P

Keff • Dec 3 '19 • Edited

Nice, I didn't know about Partial, nice find, will be useful!

Yup it could be used, but take into account that Object.assign doesn't merge nested objects.

let objA = { b: { name: 'Train' } };
let objB = { b: { description: 'A vehicle that does not like to go uphill :)' } };

let merged = Object.assign(objA, objB);
> { b: { description: "A vehicle that does not like to go uphill :)" } }

Nick Raphael • Nov 26 '19

Interesting pattern. I may have to play with it. Thanks!

Nick Taylor • Dec 2 '19 • Edited

Yeah Partial<T> is great. Another good one is Readonly<T>. It's a neat way to make things read-only, from a TS perspective even if under the hood (read JS), things are not truly read-only.

Here's a TypeScript Playground example for those interested.

For reference, here's the full list of built-in Utilitiy Types. Lots of great stuff in there.

Looking forward to your next post!

Matheus Genteluci • Nov 26 '19

It's also interesting to use the Omit<> type, which in most cases is actually a better fit because you can actually say which attibutes are not gonna be present.

Nick Raphael • Nov 26 '19

Shoutout to Fiona Tran for bringing Partial to my attention.

The Reverand • Nov 27 '19 • Edited

This a much faster way to type Pick<T,any> :) thanks!

Jesse Reese • Nov 27 '19

Partial’s are tight!

Danielg212 • Jun 25 '20

why not simply use optional parameters ?

interface Todo {
    title?: string;
    description?: string;
 }

Benjamin Ashbaugh • Mar 27 '21

Just because something should be optional in a parameter for example doesn't mean it should be optional everywhere, as would happen if you set it optional in the interface