When example

whenexample.js

'use strict';

var when = require('when');

var check = function(value) {
  // return promise here and then ready to run next expression without waiting.
  return when.promise(function(resolve, reject) {
    // simulate delay task.
    setTimeout(function() {
      if (value === 0) {
        // use reject() to throw and error.
        reject(new Error('Zero value is not allowed'));
      }
      // call resolve() to return the result.
      resolve( 'Check ' + value );
    }, 1000);
  });
};

when
  .map([1, 2, 0], function(value) {
    return check(value);
  })
  // use `then` to run next command when the promised task is already finish.
  .then(function(results) {
    console.log('The result is:', results.join(', '));
  })
  // catch error here.
  .catch(function(err) {
    console.log('There is an error:', err);
  })
  // clearing things.
  .finally(function() {
    console.log('Done!');
  });

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