Use Partial<T> when callers may leave out properties on an object’s outer level, and Required<T> when every outer-level property must be present. Use a custom DeepPartial<T> only when nested properties may also be omitted—and only after checking how that particular definition treats arrays, tuples, functions, and other special cases. Partial and Required are built-in TypeScript utilities; DeepPartial is not a built-in utility with one standard behavior.
How to choose: decide where omission is allowed
The key question is how far into the object callers may omit fields. A shallow patch often needs only top-level properties to be optional; a nested configuration merge may need omissions at multiple levels. These utilities change static types—they do not merge, validate, or otherwise change runtime values.
| Type | Built into TypeScript? | What becomes optional or required? | Best fit | Main caution |
|---|---|---|---|---|
Partial<T> |
Yes | All properties at the top level become optional. | A caller may omit some outer fields, such as in a shallow update object. | Nested properties are not made optional automatically; distinguish omission from assigning undefined. |
Required<T> |
Yes | All properties at the top level become required. | The caller must provide every outer field, including fields originally marked optional. | It may demand more fields than a particular operation needs. |
Custom DeepPartial<T> |
No | Properties become optional recursively according to the chosen definition. | Nested objects may also be only partly specified. | There is no single standard behavior; recursion can mishandle collections, tuples, unions, functions, or special object types. |
The official TypeScript utility types reference lists Partial as released in TypeScript 2.1 and Required as released in TypeScript 2.8. It documents those built-ins, not a built-in DeepPartial.
When to use Partial<T>
Use Partial<T> when a value may leave out some properties at the outer level. It is a natural fit for an API or function that accepts a shallow update, provided the update contract really allows any subset of those top-level fields.
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interface User {
name: string;
preferences: {
theme: "light" | "dark";
emailUpdates: boolean;
};
}
type UserPatch = Partial<User>;
const patch: UserPatch = {
name: "Sam",
// preferences, if supplied, must still have its complete declared shape
};Here, name and preferences are optional. But if preferences is supplied, its theme and emailUpdates fields remain required. This follows from mapped types operating on the properties at the mapped level; see the mapped types handbook.
When to use Required<T>
Use Required<T> when an operation needs every top-level property, even though the original interface permits some to be absent.
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interface DisplayOptions {
title?: string;
compact?: boolean;
}
type CompleteDisplayOptions = Required<DisplayOptions>;CompleteDisplayOptions requires both title and compact. If the operation needs only a subset, prefer a type that states that narrower contract rather than making every property required.
When a custom DeepPartial<T> is justified
Consider a recursive helper when callers may provide partial values inside nested objects as well as at the outer level—for example, if a configuration patch may change only one nested preference. TypeScript supports recursive conditional type aliases, documented in the TypeScript 4.1 release notes. That capability makes recursive definitions expressible; it does not establish one universal DeepPartial implementation.
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Before adopting or writing one, inspect the exact definition and decide what it should do with:
- Arrays and tuples: whether their elements, positions, or both can be omitted.
- Unions: whether recursion should apply to each member and preserve its distinctions.
- Functions and callable objects: whether to leave their signatures intact.
- Special object types such as class instances, maps, and sets: whether to preserve their behavior rather than recursively transforming their members.
Do not infer these details from the name. A library-provided or project-defined DeepPartial behaves according to its own type definition, and it still provides no runtime merge or validation.
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Optional properties are not always the same as explicit undefined
With exactOptionalPropertyTypes enabled, an optional property may be absent, but assigning it the value undefined is rejected unless undefined is explicitly included in that property’s declared value type. JavaScript can distinguish an absent property from a present property whose value is undefined, for example through key enumeration and property-presence checks.
The option was introduced in TypeScript 4.4, is not part of the strict family, and requires strictNullChecks. Check your project’s setting in the TSConfig reference and the TypeScript 4.4 release notes before assuming an optional property accepts explicit undefined.
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