Various Promise Pattern Demonstrations

promise-demos.js

function bad() {
  return new Promise(function(resolve, reject) {
    if (!isValid) { reject() }
  
    retrieveMenuFromDatabase(req.session.dbId, req.session.user.MenuGroup)
      .then(function(menuResult) {
        getUserFavorites(req.session.dbId, req.session.user.UserID)
          .then(function(shortCutResults) {
            // process both responses
            resolve(calculate(shortCutResults, menuResult));
          })
          .catch(function(error) {
            var values = req.allParams();
            values.errMsg = error.message;
            ResponseUtil.validateResponseFromServer(error, res, null, values);
            reject(Error(error));
          });
      })
      .catch(function(error) {
        var values = req.allParams();
        values.errMsg = error.message;
        ResponseUtil.validateResponseFromServer(error, res, null, values);
        reject(Error(error));
      });
  });
}

function good() {
  const state = {};

  if (!isValid) { return new Promise().reject(); }
  
  return retrieveMenuFromDatabase(req.session.dbId, req.session.user.MenuGroup)
    .then(function(menuResult) {
      state.menuResult = menuResult;
      return getUserFavorites(req.session.dbId, req.session.user.UserID);
    })
    .then(function(shortCutResults) {
      // do loads of stuff
      return calculate(shortCutResults, state.menuResult);
    })
    .catch(function(error) {
      var values = req.allParams();
      values.errMsg = error.message;
      ResponseUtil.validateResponseFromServer(error, res, null, values);

      throw error;
    });
}

// Nesting || BAD
function nesting() {
  return doAsyncThing()
    .then(function (value) {
      return doSecondAsyncThing(value)
        .then(function(value) {
          return doThirdAsyncThing(value)
            .then((function (value) {
              return value
            })
            .catch(function(error) {
              handleError(error)
            })
        })
        .catch(function(error) {
          handleError(error)
        })
    })
    .catch(function(error) {
      handleError(error)
    })
}
          
function flattened() {
  return doAsyncThing()
    .then(function(value) {
      return doSecondAsyncThing(value)
    })
    .then(function(value) {
      return doThirdAsyncThing(value)
    })
    .catch(function(error) {
      handleError(error)
    })
}

// notice the `throw error` line
function flattenedHandleAndPropogateError() {
  return doAsyncThing()
    .then(function(value) {
      return doSecondAsyncThing(value)
    })
    .then(function(value) {
      return doThirdAsyncThing(value)
    })
    .catch(function(error) {
      handleError(error)
      throw error
    })
}

// if you just want the error to be handled externally, don't catch it
function flattenedPropogateError() {
  return doAsyncThing()
    .then(function(value) {
      return doSecondAsyncThing(value)
    })
    .then(function(value) {
      return doThirdAsyncThing(value)
    })
}

// because we're just forwarding the argument from each promise to the next, we can go pointfree
function pointfreeChaining() {
  return doAsyncThing()
    .then(doSecondAsyncThing)
    .then(doThirdAsyncThing)
    .catch(handleError)
}

function renderTodos(todos) {
  return `<ul>
    ${todos.map(function(todo) {
      return `<li>todo.title</li>`
    })}
  </ul>`
}
 
function aRealWorldExample() {
  return fetch('https://jsonplaceholder.typicode.com/todos')
    .then(function(res) { return res.json() })
    .then(renderTodos)
    .catch(function(error) {
      throw Error(`FetchTodoError: ${error.message}`)
    })
}
  
// Use typed errors (probably imported from another file)
function aRealWorldExampleWithTypedErrors() {
  return fetch('https://jsonplaceholder.typicode.com/todos')
    .then(
      function(res) { return res.json() }, // runs if fetch succeeded
      function(error) { throw new FetchError(error.message)} // runs if fetch failed
    )
    .then(
      renderTodos, // runs if res.json succeeded
      function(error) { throw new JSONError(error.message)} // runs if res.json failed
    )
    .catch(function(error) { // runs if fetch, res.json or renderTodos failed
      if (error instanceof FetchError) { /* fetch error */ }
      if (error instanceof JSONError) { /* res.json error */  }
      if (error instanceof Error) { /* renderTodos error */ }
    })
}
 

Great stuff, thanks Lee