filter · Docs - Routecraft

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filter(fn: Filter<Current> | CallableFilter<Current>): RouteBuilder<Current>

Filter exchanges based on a predicate. The predicate receives the full Exchange object, allowing you to filter based on headers, body, or other exchange properties.

Return true to keep the exchange, false to drop it, or { reason: "..." } to drop with an explanation that is recorded in telemetry and shown in the TUI.

// Simple boolean filter
.filter((exchange) => exchange.body.isActive)

// Drop with a reason (shown in TUI traces)
.filter((exchange) => {
  if (!exchange.body.name) return { reason: "name is required" };
  if (exchange.body.age < 18) return { reason: "age must be 18 or older" };
  return true;
})

// Async filter
.filter(async (exchange) => await isValidOrder(exchange.body))

// Filter based on headers
.filter((exchange) => exchange.headers['x-priority'] === 'high')

Note

Filter vs Transform

Unlike .transform() which receives only the body, .filter() receives the full Exchange object. This allows filtering based on headers, correlation IDs, or other exchange metadata, not just the message body.