Convert array of objects to an object of arrays

Question

I have the following structure

var nights = [
    { "2016-06-25": 32, "2016-06-26": 151, "2016-06-27": null },
    { "2016-06-24": null, "2016-06-25": null, "2016-06-26": null },
    { "2016-06-26": 11, "2016-06-27": 31, "2016-06-28": 31 },
];

And I want to transform it to:

{ 
    "2016-06-24": [null], 
    "2016-06-25": [32, null], 
    "2016-06-26": [151, null, 11], 
    "2016-06-27": [null, 31], 
    "2016-06-28": [31] 
}

What's the shortest way to solve this? I have no problems with using Underscore, Lodash or jQuery.

My current code is:

var out = {};
for (var i = 0; i < nights.length; i++) {
    for (var key in nights[i]) {
        if (out[key] === undefined) {
            out[key] = [];
        }
        out[key].push(nights[i][key]);
    }
}

It's similar to Convert array of objects to object of arrays using lodash but that has all keys present in each object.

Your object has error. You can't use - in key without " or '. — Mohammad
– Mohammad, Commented Jun 23, 2016 at 10:10
@DmitriPavlutin Looping over every object in turn, creating a new one. Too messy & dirty to post here, it's an embarrassment — PeterB
– PeterB, Commented Jun 23, 2016 at 10:11

George Karanikas · Accepted Answer · 2016-06-23 10:15:09Z

5

You can do it with the following snippet (no need for lodash etc):

const x = [{ '2016-06-25': 32, '2016-06-26': 151, '2016-06-27': null }, { '2016-06-24': null, '2016-06-25': null, '2016-06-26': null }, { '2016-06-26': 11, '2016-06-27': 31, '2016-06-28': 31 }, ];
let y = {};

x.forEach(obj => {
  Object.keys(obj).forEach(key => {
    y[key] = (y[key] || []).concat([obj[key]]);
  });
});

console.log(y)

answered Jun 23, 2016 at 10:15

George Karanikas

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3 Comments

PeterB Over a year ago

When I run your snippet it gives me two errors as well as the answer (Chrome, latest)

PeterB Over a year ago

Same error, appearing twice: { "message": "Script error.", "filename": "", "lineno": 0, "colno": 0 }

George Karanikas Over a year ago

I cannot reproduce that, I'm sorry. A short google search though shows that there are other users having the same issue with the snippets in stackoverflow. Try running it in the browser's console by copy pasting. You should not get any error then (as this seems to be an issue with stackoverflow)

Vladimir Burnin · Accepted Answer · 2019-07-11 15:58:40Z

1

Here is my one-liner. Uses map to map keys into array of objects with properties of the keys. Then maps the original array to only that property and sets it as the value of the property. One issue with this way is that it will only use the properties of the first object in the array. So if other objects have properties that aren't in the first object they will be ignored.

const output = Object.assign({}, ...Object.keys(input[0]).map(props => ({[props]: input.map(prop => prop[props])})))

Edit: the output was in the wrong format, fixed

edited Jul 11, 2019 at 15:58

answered Jul 11, 2019 at 14:54

Vladimir Burnin

113 bronze badges

2 Comments

technogeek1995 Over a year ago

Welcome to SO! It's important to add some explanation to your code instead of just supplying a code-only answer.

Vladimir Burnin Over a year ago

Did my best its a little hard to explain

Nina Scholz · Accepted Answer · 2016-06-23 10:14:55Z

0

You could iterate over the array and the over the keys of the object and build a new object with the keys as new keys.

var data = [{ '2016-06-25': 32, '2016-06-26': 151, '2016-06-27': null }, { '2016-06-24': null, '2016-06-25': null, '2016-06-26': null }, { '2016-06-26': 11, '2016-06-27': 31, '2016-06-28': 31 }, ],
    result = {};

data.forEach(function (o) {
    Object.keys(o).forEach(function (k) {
        result[k] = result[k] || [];
        result[k].push(o[k]);
    });
});

console.log(result);

answered Jun 23, 2016 at 10:14

Nina Scholz

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Comments

Saad Riaz · Accepted Answer · 2019-03-05 07:53:14Z

0

Used Typescript -- obviously you can remove the types while working in JavaScript.

Assumption: The array of objects coming into the function will always have the same kind and number of object keys

mapperArrayOfJsonsToJsonOfArrays(inputArrayOfJsons: any): any {
    if (inputArrayOfJsons.length > 0) {
        let resultMap = {};
        let keys: any[] = Object.keys(inputArrayOfJsons[0]);
        keys.forEach((key: any) => {
            resultMap[key] = [];
        });

        inputArrayOfJsons.forEach((element: any) => {
            let values: any[] = Object.values(element);
            let index = 0;
            values.forEach((value: any) => {
                resultMap[keys[index]].push(value);
                index = index + 1;
            });
        });
        return resultMap;
    }
    return {};
}

edited Mar 5, 2019 at 7:53

answered Mar 5, 2019 at 5:59

Saad Riaz

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1 Comment

Jack Over a year ago

Wouldn't this remove types considering there were no generics used?

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