Create new array with two columns from old array

Question

I have an array as follows:

items = [
{"year": 2010, "month": 11, "day":23}
{"year": 2009, "month": 10, "day":15}
{"year": 2009, "month": 10, "day":10} //added after my edit below
]

I would like to create a new array with only 2 of the columns like this:

newArarry = [
{"year": 2010, "month": 11}
{"year": 2009, "month": 10}
]

Right now Im trying with .map() and its not working:

const newArray = [];
newArray.map({
    year: items.year,
    month: items.month
});

EDIT

After following one of the answers below, I realized I forgot to mention that I would also need to filter the result to be only unique rows. Now that I am only selecting the year and month columns, I am getting multiple duplicate rows

do the map on the Items and it will return to you the new array. — MennyMez, May 21 '19 at 08:25

Code Maniac · Answer 1 · 2019-05-21T08:29:33.987

7

Right now Im trying with .map() and its not working

Because newArray is having length 0 and you're trying to map on it
map accepts a callback but you're passing object
Not assigning output of map to any variable

let items = [{"year": 2010, "month": 11, "day":23},{"year": 2009, "month": 10, "day":15}]

let final = items.map(({day,...rest}) => ({...rest}))

console.log(final)

edited May 21 '19 at 08:29

answered May 21 '19 at 08:21

Code Maniac

37,143
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score 6 · Answer 2 · answered May 21 '19 at 08:25

You are not calling Array#map correctly:

You need to call in on the source array: items.
You need to provide a function as an argument.
Array#map returns a new array that is the result of the function being executed against each member of the original array.

items = [
{"year": 2010, "month": 11, "day":23},
{"year": 2009, "month": 10, "day":15}
]

const newArray = items.map(item => ({
    year: item.year,
    month: item.month
}));

console.log(newArray)

A mapping operation transforms from A to B - in this case from {"year": 2010, "month": 11, "day":23} to { "year": 2010, "month": 11 }. Running a .map on an array gives you the result of the mapping operation.

Thanks, I am currently using this method. However, I am now getting duplicate rows. Is there a way to only select unique rows (my items array is much larger than whats in my question) — Mennyg, May 21 '19 at 08:53
@Mennyg have a look [at this answer](https://stackoverflow.com/a/9229821/3689450) — VLAZ, May 21 '19 at 08:56

score 5 · Answer 3 · answered May 21 '19 at 08:21

Array's .map() method accepts a callback whose returned value will be used to construct new array. Mistakenly you are passing it an object instead of a function. To fix this you can use .map() with some Object Destructuring:

const items = [
  {"year": 2010, "month": 11, "day":23},
  {"year": 2009, "month": 10, "day":15}
];

const result = items.map(({ year, month }) => ({year, month}));

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Jack · Answer 4 · 2019-05-21T12:43:49.260

Array.map() expects a callback as the first parameter, it must also be called on the original array - otherwise you're essentially iterating over an empty array.

var newArray = items.map(function (item) {
    return {
        year: item.year,
        month: item.month
    };
});

Update

In order to make sure your array only contains unique values, rather than mapping and filtering, you could do something similar to the following:

var items = [
    {year: 2010, month: 11, day: 23},
    {year: 2009, month: 10, day: 15},
    {year: 2009, month: 10, day: 10}
];

var newArray = [];

items.forEach(function (item) {
    var index = newArray.findIndex(function (newItem) {
        return newItem.year === item.year && newItem.month === item.month;
    });

    if (index === -1) {
        newArray.push({ year: item.year, month: item.month });
    }
});

console.log(newArray); // [{ year: 2010, month: 11 }, { year: 2009, month: 10 }]

score 2 · Answer 5 · answered May 21 '19 at 08:30

You're not using map correctly - you can also simplify with rest, spread, and destructuring:

const items = [{"year": 2010, "month": 11, "day":23},{"year": 2009, "month": 10, "day":15}];
const newArray = items.map(({ day, ...res }) => ({ ...res }));
console.log(newArray);

.as-console-wrapper { max-height: 100% !important; top: auto; }

Dilip Chauhan · Answer 6 · 2019-05-21T11:18:19.477

1

Correct your items array, Array with multiple elements/values must be comma separated

var items = [{"year": 2010, "month": 11, "day":23},
               {"year": 2009, "month": 10, "day":15}]
                    const newArray = [];
                    items.map(i=>{
                    newArray.push({year: i.year, month: i.month});
                    });
                  console.log(newArray);

edited May 21 '19 at 11:18

answered May 21 '19 at 08:29

Dilip Chauhan

147
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10

Create new array with two columns from old array

6 Answers6