Promises chaining

1- Your snippet (callback style).js

promiseA.then(function (resultA) {
  promiseB.then(function (resultB) {
    object.method(arg1, arg2, function (resultC) {
      console.log(resultC);
    });
  });
});

2- Your snippet (promise style).js

promiseA.then(function (resultA) {
  return promiseB;
}).then(function (resultB) {
  object.method(arg1, arg2, function (resultC) {
    console.log(resultC);
  });
});

3- Your snippet (promise style, a bit improved).js

promiseA.then(function (resultA) {
  return promiseB;
}).then(function (resultB) {
  // Now returns a promise
  return object.method(arg1, arg2);
}).then(function (resultC) {
  console.log(resultC);
});

4- Why?.js

// What about more than 1 or 2 nesting levels?

promiseA.then(function (resultA) {
  promiseB.then(function (resultB) {
    object.method(arg1, arg2, function (resultC) {
      promiseC.then(function (resultC) {
        promiseD.then(function (resultD) {
          // And so on...
        });
      });
    });
  });
});

// Promise style

promiseA.then(function (resultA) {
  return promiseB;
}).then(function (resultB) {
  // Now returns a promise
  return object.method(arg1, arg2);
}).then(function (resultC) {
  return promiseD;
}).then(function (resultD) {
  // And so on...
  // Easier/better composition
});

5- Bonus: error handling.js

// How do we handle errors?

promiseA.then(function (resultA) {
  promiseB.then(function (resultB) {
    object.method(arg1, arg2, function (resultC) {
      promiseC.then(function (resultC) {
        promiseD.then(function (resultD) {
          // And so on...
        }).catch(function (error) {
          // Handles promiseD errors only
        });
      }).catch(function (error) {
        // Handles promiseC errors only
      });
    });
  }).catch(function (error) {
    // Handles promiseB errors only
  });
}).catch(function (error) {
  // Handles promiseA errors only
});

// Promise style

promiseA.then(function (resultA) {
  return promiseB;
}).then(function (resultB) {
  // Now returns a promise
  return object.method(arg1, arg2);
}).then(function (resultC) {
  return promiseD;
}).then(function (resultD) {
  // And so on...
  // Easier/better composition
}).catch(function (error) {
  // Handles all errors at the end of the chain
});