promise-condition

Rejects a promise that fails a test condition

var condition = require('@songkick/promise-condition');
var rejectAbove400 = condition(function (data) { return data.status < 400; });

function fetch200(){
    return fetch('/path/to/a/http-200-ok');
}

function fetch400(){
    return fetch('/path/to/a/http-400-bad-request');
}

rejectAbove400(fetch200)()
  .then(function(response){
    // the initial fetch response
  }).catch(function(err){
    // probably won't happen here, unless /200 doesn't return a HTTP - 200
  });

rejectAbove400(fetch400)()
  .then(function(response){
    // should not happen here
  }).catch(function(err){
      // err instanceof condition.ConditionError === true
      // err === {
      //   message: 'Condition test failed',
      //   fn: fetch400,
      //   data: // the original data
      // }
  });

Composition

As promise-condition input and output is a function returning a promise, you can compose them easily with other similar helpers (see below).

In the example below, our /data API is a bit janky and might return HTTP 500 errors. We'll retry them twice before giving up.

var promiseRetry = require('@songkick/promise-retry');
var promiseCondition = require('@songkick/promise-condition');

var retryTwice = promiseRetry({ retries: 2 });
var rejectAbove500 = promiseCondition(function (data) {
	return data.status < 500;
});

function fetchData() {
    // this call might return 500 sometimes
    return fetch('/data');
}

retryTwice(rejectAbove500(fetchData))().then(function(response){
  // yay !
}).catch(function(err){
  // we retried the call twice but always got 500s :(
});

See also

promise-condition composes really well with the following promise helper:

• promise-retry
• promise-timeout
• promise-condition