JavaScript with Promises examples

_general_mixing_sync_and_async.js

Promise.resolve().then(function() {
  console.log('starting');
  return 123;
}).then(function(val) {
  console.log(val);
  return val + 1;
}).then(function(val) {
  console.log('starting second step...');
  return new Promise(function(resolve) {
    setTimeout(function() {
      console.log('finished second step');
      resolve(val + 1);
    }, 500);
  });
}).then(function() {
  console.log('done');
});

// starting
// 123
// starting second step...
// finished second step
// done

conditional_async_1.js

var loggedIn = false;

var login = function() {
  return new Promise(function(resolve) {
    setTimeout(function() {
      loggedIn = true;
      console.log("logged in");
      resolve('success');
    }, 500); 
  })
};

var display = function() {
  var promise = loggedIn ? Promise.resolve() : login();
  
  promise.then(function() {
    console.log('here is your secret');
  });
};


display();
setTimeout(display, 1000)

conditional_async_2.js

var authenticator = {
  promise: undefined,
  login: function() {
    if (this.promise === undefined) {
      this.promise = new Promise(function(resolve, reject) {
        setTimeout(function() {
          console.log("logged in");
          resolve('success');
        }, 500);
      });
    } else {
      console.log('already logged in');
    }
    
    return this.promise;
  }
};

var display = function() {
  authenticator.login().then(function() {
    console.log("here's your account");
  });
};

display();
setTimeout(display, 1000);

parallel_execution.js

var _ = require('underscore');

var accounts = ['Checking Account', 'Travel Rewards Card', 'Big Box Retail Card'];

var promises = _.map(accounts, function(account) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      console.log("checked " + account);
      resolve();
    }, 1000)
  });
});

Promise.all(promises).then(function() {
  console.log("Checked all");
});

sequential_execution.js

var _ =  require('underscore');

var sequence = function(products) {
  _.reduce(products, function(promise, product) {
    return promise.then(function() {
      return queryFn(product);
    })
  }, Promise.resolve());
};

var queryFn = function(product) {
  return new Promise(function(resolve, reject) {
    console.log("querying " + product);
    setTimeout(function() {
      var result = product + " checked.";
      console.log(result);
      resolve(product + " checked.");
    }, 1000);   
  });
  
};

var products = ["a", "b", "c"];

sequence(products);

sequential_execution_recursion.js

var _ = require('underscore');

var products = ['a', 'b', 'c'];

var queryFn = function(product) {
  return new Promise(function(resolve) {
    console.log('checking ' + product);
    
    setTimeout(function() {
      console.log('checked ' + product);
      resolve();
    }, 500);
  });
};

var sequence = function(array) {
  var chain;
  
  chain = function(array, index) {
    if (array.length === index) {
      return Promise.resolve();
    } else {
      return queryFn(array[index]).then(function() {
        return chain(array, index + 1);
      })
    }
  };
  
  chain(array, 0);
};

sequence(products);