I want to iterate through an array, run a calculation, and if the condition is true for the result, return a new object. _.filter(...) would not work here, since the iterator function must return either true or false.

_.map(people, function(person) {

    var age = calculateAge(person.birthDate);

    if (age > 50) {
        return {person: person, age: age};
    }

});

I've tried searching all over, including the documentation, but I haven't found a way to do this well.

I want to iterate through an array, run a calculation, and if the condition is true for the result, return a new object. _.filter(...) would not work here, since the iterator function must return either true or false.

_.map(people, function(person) {

    var age = calculateAge(person.birthDate);

    if (age > 50) {
        return {person: person, age: age};
    }

});

I've tried searching all over, including the documentation, but I haven't found a way to do this well.

• javascript
• lodash

3 Answers 3

Sounds like maybe you want reduce and not map:

var newArray = _.reduce(people, function(results, person) {
  var age = calculateAge(person.birthDate);
  if (age > 50) {
     results.push({ person: person, age: age });
  }
  return results;
}, []);

Also if you are ES6+ and/or using Babel, this might be a good use for list prehensions:

let newArray = [for (person of people)
                if (calculateAge(person.birthDate) > 50)
                { person: person, age: calculateAge(person.birthDate) }
               ];

Update: List prehensions have been dropped from from Babel 6. The ES2015 version would look like:

const newArray = people.reduce((results, person) => {
  const age = calculateAge(person.birthDate);
  return (age > 50) ? [...results, { person, age }] : results;
}, []);

(https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator)

This could work for you:

var sorted = _.filter(people, function(person) {

    var age = calculateAge(person.birthDate);

    if (age > 50) {
      return true;
    }
});

var newArray = _.map(sorted, function(person) {

   var age = calculateAge(person.birthDate);

   return {person: person, age: age};
});

This will first filter the list and then return a new array with the objects of the people whos age is above 50.

Try this:

people = _.pluck(
                _.filter(
                        _.map(people, function(person) {
                            return { person: person, age: calculateAge(person.birthDate) };
                        }), 
                        function(person) {
                            return person.age > 50;
                        }
                ), 
                "person"
        );

See working fiddle