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Call the supplied function exactly one time. This prevents double callback execution. This module can be used on both node and browsers using browserify. No magical ES5/6 methods used unlike the once module does.


npm install one-time


Simply supply the function with the function that should only be called one time:

var one = require('one-time');
function load(file, fn) {
  fn = one(fn);
  eventemitter.once('load', fn);
  eventemitter.once('error', fn);
  // do stuff 
  eventemitter.emit('error', new Error('Failed to load, but still finished'));
function example(fn) {
  fn = one(fn);
  fn('also receives all arguments');
  fn('it returns the same value') === 'bar';
example(function () { 
  return 'bar'

Why not once?

The main reason is that once cannot be used in a browser environment unless it's ES5 compatible. For a module as simple as this I find that unacceptable. In addition to that it super heavy on the dependency side. So it's totally not suitable to be used in client side applications.

In addition to that we make sure that your code stays easy to debug as returned functions are named in the same way as your supplied functions. Making heap inspection and stacktraces easier to understand.