try expressions

Proposal for simpler error handling
when using functions and Promises in ECMAScript

Summary

The current try...catch error handling pattern leads to uncaught exceptions when dealing with promises. This is a new try syntax to combine a Promise's value and error or wrap an expression in a try...catch with a single return type.

let [json, jsonErr] = try JSON.parse('{}')
let [value, err] = try await fetch()

// instead of

let json, jsonErr, value, err

try {
  json = JSON.parse('{}')
} catch (_jsonErr) {
  jsonErr = _jsonErr
}

try {
  value = await fetch()
} catch (_err) {
  err = _err
}

It's backwards-compatible with existing try...catch usage, functions and expressions.

Return Type

interface TryExpressionReturn {
  [0]: any
  [1]: Error
}

Example

async function fetchUser(userId) {
  let [gqlRes, gqlErr] = try await fetch(GRAPHQL_HOST + queryForUser(userId))
  
  if (!gqlErr) return gqlRes
  
  // If the GraphQL server doesn't respond properly, use fallback API
  let [apiRes, apiErr] = try await fetch(`${V1_API_HOST}/v1/users/${userId}`)
  
  // If the fallback API fails, throw the error
  if (apiErr) throw apiErr
  
  return apiRes
}

// Or for error reporting

let [, err] = try importantFn()

reprotError(err)
reportError((try importantFn())[1])

Design Decisions

• Array return type. This removes unnecessary opinion and bytes from source compared to using an Object.
  • 👍 let [myValue, myError] = try await fn()
  • 👎 let { value: myValue, error: myError } = try await fn()
• Array order inspired by Golang. This seems to make more sense instead of callback-style with error as the first argument, e.g. res, err := http.Get(API_HOST)

Alternatives

Status Quo

async/await

async function fetchUser(userId) {
  try {
    return await fetch(GRAPHQL_HOST + queryForUser(userId))
  } catch (gqlErr) {
    try {
      return await fetch(`${V1_API_HOST}/v1/users/${userId}`)
    } catch (apiErr) {
      throw apiErr
    }
  }
}

async/await + catch

async function fetchUser(userId) {
  let gqlErr, gqlErr

  gqlRes = await fetch(GRAPHQL_HOST + queryForUser(userId)).catch(err => gqlErr = err)
  
  if (gqlErr) {
    return await fetch(`${V1_API_HOST}/v1/users/${userId}`)
  }
  
  return gqlRes
}

Userland Modules

• await-to-js
• @nucleartide/dx

Adoption Strategy

• This proposal is implemented as a Babel plugin to compile try expressions down to valid ECMAScript
• Backwards-compatible with existing exception handling and Promise spec
• Open to community for comment and testing

Acknowledgements

• Zack Argyle