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Overview

Chains operations on a ResultAsync, where the next operation depends on the success of the previous one. The callback must return a Result or ResultAsync.

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Signature

class ResultAsync<T, E> {
  andThen<U, F>(
    f: (t: T) => Result<U, F> | ResultAsync<U, F>
  ): ResultAsync<U, E | F>
}
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f
(t: T) => Result<U, F> | ResultAsync<U, F>
required
Function that takes the Ok value and returns a new Result or ResultAsync
Returns: ResultAsync<U, E | F> - A new ResultAsync with combined error types

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Usage

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Sequential async operations

import { ResultAsync } from 'neverthrow'

const processUser = (id: number) => {
  return fetchUser(id)              // ResultAsync<User, FetchError>
    .andThen(validateUser)           // Result<User, ValidationError>
    .andThen(saveUser)               // ResultAsync<User, SaveError>
    .andThen(sendNotification)       // ResultAsync<void, NotificationError>
}

// Type: ResultAsync<void, FetchError | ValidationError | SaveError | NotificationError>

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Mixing sync and async

const result = await fetchData()     // ResultAsync<Data, Error>
  .andThen(data => {                 // Sync Result
    if (data.valid) {
      return ok(data)
    }
    return err('Invalid data')
  })
  .andThen(processData)              // ResultAsync<Processed, Error>

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Flattening nested Results

// Without andThen - nested ResultAsync
const nested: ResultAsync<ResultAsync<Data, Error>, Error> = 
  okAsync(fetchData())

// With andThen - flattened
const flat: ResultAsync<Data, Error> = 
  okAsync(fetchData()).andThen(data => data)

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Key characteristics

Unlike map, andThen can introduce new error types. The resulting error type is the union of all possible errors.
  • Error propagation: Short-circuits if any operation fails
  • Type unions: Combines error types from all operations
  • Flexible: Accepts both sync Result and async ResultAsync

map

Transform without changing Result structure

orElse

Handle errors in the chain

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