Using es6 javascript is it possible to iterate over an object and return a new object. For example:

const people = {
    'Sally': {
        age: 22,
        sex: 'female',
    },
    'John': {
        age: 64,
        sex: 'male',
    },
    'Sam': {
        age: 12,
        sex: 'female',
    },      
};

const ages = people.someEs6IteratingObjectFunction((index, person) => {
    return { Object.keys(people)[index]: person.age };
});

console.log(ages); // { 'Sally': 22, 'John': 64, 'Sam': 12, }   

Using es6 javascript is it possible to iterate over an object and return a new object. For example:

const people = {
    'Sally': {
        age: 22,
        sex: 'female',
    },
    'John': {
        age: 64,
        sex: 'male',
    },
    'Sam': {
        age: 12,
        sex: 'female',
    },      
};

const ages = people.someEs6IteratingObjectFunction((index, person) => {
    return { Object.keys(people)[index]: person.age };
});

console.log(ages); // { 'Sally': 22, 'John': 64, 'Sam': 12, }   

• javascript
• iterator
• ecmascript-6

5 Answers 5

You can use reduce method of array prototype. It also works in es5.

const people = {
    'Sally': {
        age: 22,
        sex: 'female',
    },
    'John': {
        age: 64,
        sex: 'male',
    },
    'Sam': {
        age: 12,
        sex: 'female',
    },      
};

let result = Object.keys(people).reduce(function(r, name) {
  return r[name] = people[name].age, r;
}, {});

document.write(['<pre>', JSON.stringify(result, 0, 3), '</pre>'].join(''));

One option if you're not super concerned about potential performance:

const ages = people.reduce(
    (obj, name) => Object.assign(obj, {[name]: obj[name].age}), {});

or maybe

const ages = Object.entries(people).reduce(
    (obj, [name, info]) => Object.assign(obj, {[name]: info.age}), {});

Or make an ES6 Map instead of using an Object:

const ages = new Map(Object.entries(people).map(
    ([name, info]) => [name, info.age])))

Here is one way to do it, I wish I could take credit for this, it is from Axel Rauschmayer's blog, he has several other amazing blog posts here: http://www.2ality./2015/02/es6-iteration.html

Here is a JSBin with the code below:

 function objectEntries(obj) {
        let index = 0;
        let propKeys = Reflect.ownKeys(obj);
        return {
            [Symbol.iterator]() {
                return this;
            },
            next() {
                if (index < propKeys.length) {
                    let key = propKeys[index];
                    index++;
                    return { value: [key, obj[key]] };
                } else {
                    return { done: true };
                }
            }
        };
    }

    let obj = { first: 'Jane', last: 'Doe' };
    for (let [key,value] of objectEntries(obj)) {
        console.log(`${key}: ${value}`);
    }

This is even easier if you use generators: http://www.2ality./2015/03/es6-generators.html

Unfortunately, there's isn't an equivalent of Array.map for objects in ES6.

Maybe you can make a helper function to mimic this. It would iterate over your object, calling the callback for each key with the value at that key, and replacing that value with whatever the callback returns. Something like:

const objectMap = (obj, cb) => 
  Object.keys(obj).reduce((ppl, k) =>
    Object.assign(ppl, {
      [k]: cb(obj[k])
    }), {}
  );

And then use it like:

const newPeople = objectMap(people, val => val.age);
console.log(newPeople); // Object {Sally: 22, John: 64, Sam: 12}

Running example:

const people = {
  'Sally': {
    age: 22,
    sex: 'female',
  },
  'John': {
    age: 64,
    sex: 'male',
  },
  'Sam': {
    age: 12,
    sex: 'female',
  },
};

const objectMap = (obj, cb) => 
  Object
  .keys(obj)
  .reduce(
    (ppl, k) =>
    Object.assign(ppl, {
      [k]: cb(obj[k])
    }), {}
  );

const newPeople = objectMap(people, val => val.age);

console.log('---- People ----');
console.dir(people);

console.log('---- New People ---- ');
console.dir(newPeople);

Maybe you can use for...of?

 const people = { 'Sally': { age: 22, sex: 'female', }, 'John': { age: 64, sex: 'male', }, 'Sam': { age: 12, sex: 'female', },
};

const ages = {};
for(let index of people) {
  ages[key] = people[key].age;
}
console.log(ages); // { 'Sally': 22, 'John': 64, 'Sam': 12, }