defineProperty

Defines a computed property on each item.

Signature

defineProperty<K extends string, U>(
  propertyName: K,
  compute: (item: CurrentItem) => U
): PipelineBuilder</* type with K: U added */>

Parameters

Parameter	Type	Description
propertyName	string	Name of the new property
compute	function	Function to compute the property value

Returns

A new PipelineBuilder with the computed property added to items.

Example

interface Person {
  firstName: string;
  lastName: string;
  birthYear: number;
}

const builder = createPipeline<Person>()
  .defineProperty("fullName", p => `${p.firstName} ${p.lastName}`)
  .defineProperty("age", p => new Date().getFullYear() - p.birthYear);

Result:

{
  firstName: "John",
  lastName: "Doe",
  birthYear: 1990,
  fullName: "John Doe",  // computed
  age: 34                // computed
}

Use Cases

• Concatenating fields - Full names, addresses
• Computing derived values - Age from birth year, duration from timestamps
• Formatting - Currency strings, date formats
• Categorization - Bucket values into ranges

With Grouping

Use before groupBy to create custom grouping keys:

const builder = createPipeline<Event>()
  .defineProperty("hourBucket", e => Math.floor(e.timestamp / 3600000))
  .groupBy(["hourBucket"], "byHour");

Scoped Usage

Add computed properties at nested levels:

const builder = createPipeline<Order>()
  .groupBy(["category"], "categories")
  .in("categories").defineProperty(
    "itemTotal", 
    item => item.quantity * item.unitPrice
  );

See Also

• dropProperty - Remove properties