Conditional and repeated function invocation for node and browser.



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Because everyone loves a tryer! Conditional and repeated function invocation for node and browser.

Say what?

Sometimes, you want to defer calling a function until a certain pre-requisite condition is met. Other times, you want to call a function repeatedly until some post-requisite condition is satisfied. Occasionally, you might even want to do both for the same function.

To save you writing explicit conditions and loops on each of those occasions, tryer implements a predicate-based approach that hides the cruft behind a simple, functional interface.

Additionally, it allows you to easily specify retry intervals and limits, so that your code doesn't hog the CPU. It also supports exponential backoff of retry intervals, which can be useful when handling indefinite error states such as network failure.

What size is it?

5.6 kb unminified with comments, 1.1 kb minified, 0.5 kb minified + gzipped.

How do I install it?

Via npm:

npm i tryer --save

Or if you just want the git repo:

git clone

How do I use it?

Loading the library

If you are running in Node.js or another CommonJS-style environment, you can require tryer like so:

const tryer = require('tryer');

It also the supports the AMD-style format preferred by Require.js.

If you are including tryer with an HTML <script> tag, or neither of the above environments are detected, it will be exported globally as tryer.

Calling the exported function

tryer is a function that can be invoked to call other functions conditionally and repeatedly, without the need for explicit if statements or loops in your own code.

tryer takes one argument, an options object that supports the following properties:

  • action: The function that you want to invoke. If action returns a promise, iterations will not end until the promise is resolved or rejected. Alternatively, action may take a callback argument, done, to signal that it is asynchronous. In that case, you are responsible for calling done when the action is finished. If action is not set, it defaults to an empty function.

  • when: A predicate that tests the pre-condition for invoking action. Until when returns true (or a truthy value), action will not be called. Defaults to a function that immediately returns true.

  • until: A predicate that tests the post-condition for invoking action. After until returns true (or a truthy value), action will no longer be called. Defaults to a function that immediately returns true.

  • fail: The error handler. A function that will be called if limit falsey values are returned by when or until. Defaults to an empty function.

  • pass: Success handler. A function that will be called after until has returned truthily. Defaults to an empty function.

  • limit: Failure limit, representing the maximum number of falsey returns from when or until that will be permitted before invocation is deemed to have failed. A negative number indicates that the attempt should never fail, instead continuing for as long as when and until have returned truthy values. Defaults to -1.

  • interval: The retry interval, in milliseconds. A negative number indicates that each subsequent retry should wait for twice the interval from the preceding iteration (i.e. exponential backoff). The default value is -1000, signifying that the initial retry interval should be one second and that each subsequent attempt should wait for double the length of the previous interval.


// Attempt to insert a database record, waiting until `db.isConnected`
// before doing so. The retry interval is 1 second on each iteration
// and the call will fail after 10 attempts.
  action: () => db.insert(record),
  when: () => db.isConnected,
  interval: 1000,
  limit: 10,
  fail () {
    log.error('No database connection, terminating.');
// Attempt to send an email message, optionally retrying with
// exponential backoff starting at 1 second. Continue to make
// attempts indefinitely until the call succeeds.
let sent = false;
  action (done) {
    smtp.send(email, error => {
      if (! error) {
        sent = true;
  until: () => sent,
  interval: -1000,
  limit: -1
// Poll a device at 30-second intervals, continuing indefinitely.
  action: () => device.poll().then(response => handle(response)),
  interval: 30000,
  limit: -1

How do I set up the dev environment?

The dev environment relies on Chai, JSHint, Mocha, please-release-me, spooks.js and UglifyJS. The source code is in src/tryer.js and the unit tests are in test/unit.js.

To install the dependencies:

npm i

To run the tests:

npm t

To lint the code:

npm run lint

To regenerate the minified lib:

npm run minify

What license is it released under?