A fluent builder allows chaining calls to the methods that set properties:
interface ExampleType {
typeName: string;
id: number;
isBest: boolean;
}
const exampleBuilder: Builder<ExampleType> = createBuilder<ExampleType>();
const finishedObject = exampleBuilder.setTypeName("goodbye").setId(2).setIsBest(true).build();
The builder API usually allows for multiple calls to the same setter method, as well as calling the build()
method before all required properties have been set, resulting in a runtime error.
We can leverage typescript's type system to produce a builder type that:
- Only allows setting each property once
- Only allows calling
build()
once every required property has been set
const exampleBuilder: Builder<ExampleType> = createBuilder<ExampleType>();
exampleBuilder.build();
~~~~~~~
// Property 'build' does not exist on type 'Builder<ExampleType>'.
exampleBuilder.setId(2).setId(1);
~~~~~~~~
// Property 'setId' does not exist on type 'BuilderAux<ExampleType, "id">'
We can also create partial objects, either with a loose Partial<T>
type or with a stricter type that tracks which properties have already been set.
Partial object:
const exampleBuilder: Builder<ExampleType> = createBuilder<ExampleType>();
const partial: Partial<ExampleType> = exampleBuilder.buildPartial();
Partial object with stricter typing:
const exampleBuilder: Builder<ExampleType> = createBuilder<ExampleType>();
const builderAfterNameSet = exampleBuilder.setTypeName("hello");
const partialTyped: Pick<ExampleType, "typeName"> = builderAfterNameSet.buildPartialTyped();