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retry a function with a series of arguments until one works


retry a function with a series of arguments until one works


// run this in the repo: `npm install; node sample.js`
// test a list of servers for a response
var fallback = require('fallback')
var request = require('request')

var servers = ['http://foo.baz', '', 'http://fail']

fallback(servers, function (server, callback) {
  console.log('trying server at ' + server)
  request(server, function (err, response) {
    if (err || response.statusCode >= 400) {
      // try the next server
      return callback()
    callback(null, response.statusCode)
}, function (err, result, server) {
  if (err) {
  if (result) {
    console.log('server ' + server + ' returned: ' + result)
  } else {
    console.log('no servers returned successfully')


trying server at http://foo.baz
trying server at
server returned: 200


fallback: (array: Array, iteratorFunction: IteratorFunction, outerCallback: OuterCallback) => void

Call fallback with an array of alternative values to be used as arguments on iteratorFunction.

IteratorFunction: (arrayItem, callback: (err: Error, result) => void) => void

iteratorFunction is an async function of function (arrayItem, callback), where callback is a normal node-style callback(err, result) continuation. Note that if an error is given to callback, the entire fallback sequence will terminate early. This should be used for unrecoverable errors. To indicate that the operation on the current arrayItem did not succeed and that the next one should be tried, callback should be invoked with a null error and an undefined or false value for result. A result value of null has the semantics that "the operation succeeded (and therefore further fallbacks should not be tried), and there was no result value".

OuterCallback: (err: Error, result, arrayItem, array: Array) => void

In outerCallback, there are three possible return states: err is not undefined: there was an unrecoverable error when executing the fallback sequence. result is false: none of the fallback alternatives were successful. arrayItem is null, and array contains the original array. result is not false: one of the fallback alternatives was successful. result contains the result value of that operation and arrayItem contains the value that was used in the successful operation. array contains the original array.

The parameters for outerCallback are similar to those in the callback for - first the value, then an index, then the original collection.

isn't this the same as async.some?

Sort of, but fallback has much better semantics, and it uses normal node-style async callbacks (eg, callback(err, result) ) for composability.


with npm

$ npm install fallback



MIT (c) MMXIII jden - Jason Denizac