如何使承诺更加易读？

Question

I have the following code. Is there a better way to write it. It feels like when conditions and promises and querying from multiple tables are involved, the code becomes harder to read. Any help would be appreciated. Thanks!

fetchUserById = function (id) {
    var user = {};

    return knex_instance('user_info')
      .where('id', id)
      .first()
      .then(function (data) {
        if (!data) {
          return null;
        } else {
          user.info = data;
          return knex_instance('user_table')
            .where('id', id)
            .first()
            .then(function (values) {
              user.values = values;
              return user;
            });
        }
      })
      .catch(errorHandler('fetchUserById', id));
  }

Answer 1

Instead of increasing the indentation level with each request, you can return the request to the original promise chain and add a then there. Like this:

fetchUserById = function(id) {
  var user = {};

  return knex_instance('user_info').where('id', id).first()
    .then(function(data) {
      if (!data) 
        return null;
      user.info = data;
      return knex_instance('user_table').where('id', id).first();
    })
    .then(function(values) {
      if (!values) 
        return null;
      user.values = values;
      return user;
    });
    .catch(errorHandler('fetchUserById', id));
}

Problem with this approach, the null that can be returned by the first then must be passed through the second one (and the third, fourth, etc, if you had some). An alternate method would have been to use exceptions instead of returning null:

fetchUserById = function(id) {
  var user = {};

  return knex_instance('user_info').where('id', id).first()
    .then(function(data) {
      if (!data) 
        throw "NO_USER_INFO";
      user.info = data;
      return knex_instance('user_table').where('id', id).first();
    })
    .then(function(values) {
      user.values = values;
      return user;
    });
    .catch(function(err) {
      if (err==="NO_USER_INFO")
        return null;
      else
        return errorHandler('fetchUserById', id);
    }
}

Answer 2

One way to increase readability is to separate part of the code into its own function.

I don't know what this function does, so I'm just going to call it doSomething . If you replace that with something descriptive it's easier to read in two places: The function itself describes the code it contains, and the call to the function indicates what's happening at that step in the calling code.

function doSomething(data)
{
    if (!data) {
        return null;
      } else {
        user.info = data;
        return knex_instance('user_table')
          .where('id', id)
          .first()
          .then(function (values) {
            user.values = values;
            return user;
      }
}

fetchUserById = function (id) {
  var user = {};

  return knex_instance('user_info')
    .where('id', id)
    .first()
    .then(doSomething)
    .catch(errorHandler('fetchUserById', id));
}