typescript-advanced-utility-types

TypeScript Utility Types

The standard library of type transformations: which built-in utility type fits a given shape change, and how to build a custom one when none does.

Mindset

Reach for a built-in utility type before writing a custom one — Partial<T>, Pick<T, K>, ReturnType<typeof fn> and their relatives cover the overwhelming majority of shape transformations, and a hand-rolled equivalent is one more thing to keep in sync with the type it derives from. When a built-in genuinely doesn't fit (a transform needs to recurse into nested objects, or apply a key-by-key rename), build the custom version as a mapped type over the real source type rather than a parallel interface maintained by hand. Combine utilities rather than inventing a new named type for every combination: Partial<Pick<T, K>> is more honest about what it does than a bespoke PartialSubset<T, K> that hides the same composition behind a name.

When to Apply

Use this skill when:

• Making some or all properties of a type optional or required (Partial, Required)
• Selecting or excluding specific properties (Pick, Omit)
• Making a type read-only or building a lookup type (Readonly, Record)
• Filtering members out of or into a union (Extract, Exclude)
• Stripping null/undefined or unwrapping a Promise (NonNullable, Awaited)
• Extracting a function's return type, parameters, constructor parameters, or instance type
• Remapping object keys in a mapped type with an as clause
• Building a custom mapped type because no built-in utility fits

Use When

• "Is there a built-in utility type for this, or do I need to write my own?"
• "How do I extract just the parameter types from this function signature?"
• "How do I make every property in this type optional except one?"
• "How do I rename or filter keys while mapping over a type?"

Scope

In Scope

• Every TypeScript built-in utility type (Partial, Required, Pick, Omit, Readonly, Record, Extract, Exclude, NonNullable, Awaited, ReturnType, Parameters, ConstructorParameters, InstanceType, ThisParameterType, OmitThisParameter).
• Key remapping in mapped types (as clauses, key filtering).
• Custom mapped types for transforms no built-in covers.