Restructure JavaScript Array of Objects by another predefined Array element as key

Question

I put the same code also on jsbin: https://jsbin.com/literefeqo/edit?js,console

Explanation

I’ve got an array of objects (1) and would like to transform (possibly using map) this object. The transformation criteria is a given array (2) and corresponds to the german property in arrObj. That means, if there is a german property in arrObj it should be "copied" out and it should be used as a key to produce the resultObj (3). If there is no german property then a key should be "Unknown" or whatever.

Note: There can be more entries in resultObj for e.g. Montag. Thats way resultObj.Montag[i] should be an array of objects.

(1) Array of Objects

const arrObj = [
{
    "day": {
    "string": "Monday",
    "Number": 1
    },
    "description": {
    "type": "string",
    "value": "The first day of the week"
    },
    "german": {
    "type": "string",
    "value": "Montag"
    }
},
{
    "day": {
    "string": "Tuesday",
    "Number": 2
    },
    "description": {
    "type": "string",
    "value": "The second day of the week"
    }
},
{
    "day": {
    "string": "Wednesday",
    "Number": 3
    },
    "description": {
    "type": "string",
    "value": "The third day of the week"
    },
    "german": {
    "type": "string",
    "value": "Mittwoch"
    }
}
];

(2) Array that should become the key for the new object

const germanDays = ["Montag","Dienstag","Mittwoch","Donnerstag"];

(3) Result should look like

const resultObj =   {
"Montag": [
  {
    "day": {
      "string": "Monday",
      "Number": 1
    },
    "description": {
      "type": "string",
      "value": "The first day of the week"
    },
    "german": {
      "type": "string",
      "value": "Montag"
    }
  }
],
"Dienstag": [
  {}
],
"Mittwoch": [
  {
    "day": {
      "string": "Wednesday",
      "Number": 3
    },
    "description": {
      "type": "string",
      "value": "The third day of the week"
    },
    "german": {
      "type": "string",
      "value": "Mittwoch"
    }
  }
],
"Donnerstag": [
  {}
],
"Unknown": [
  {
    "day": {
      "string": "Tuesday",
      "Number": 2
    },
    "description": {
      "type": "string",
      "value": "The second day of the week"
    }
  }
]

};

And you know that object properties do not have order in JavaScript? Just checking... — VisioN
– VisioN, Commented Dec 3, 2018 at 13:59

Keith · Accepted Answer · 2018-12-03 16:28:43Z

3

(possibly using map)

map, is meant for Array to Array mapping, a more appropriate function is reduce.

Here is an example.

const germanDays = ["Montag","Dienstag","Mittwoch","Donnerstag"]

const arrObj = [
{
    "day": {
    "string": "Monday",
    "Number": 1
    },
    "description": {
    "type": "string",
    "value": "The first day of the week"
    },
    "german": {
    "type": "string",
    "value": "Montag"
    }
},
{
    "day": {
    "string": "Tuesday",
    "Number": 2
    },
    "description": {
    "type": "string",
    "value": "The second day of the week"
    }
},
{
    "day": {
    "string": "Wednesday",
    "Number": 3
    },
    "description": {
    "type": "string",
    "value": "The third day of the week"
    },
    "german": {
    "type": "string",
    "value": "Mittwoch"
    }
},
{
    "day": {
    "string": "Monday",
    "Number": 1
    },
    "description": {
    "type": "string",
    "value": "Just another text is here"
    },
    "german": {
    "type": "string",
    "value": "Montag"
    }
}
];

const ret = germanDays.reduce((a, v) => {
  const f = arrObj.filter(f => f.german && f.german.value === v);
  a[v] = f;
  return a;
}, {
  "Unknown": arrObj.filter(f => !f.german)
});


console.log(ret);

edited Dec 3, 2018 at 16:28

answered Dec 3, 2018 at 14:12

Keith

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4 Comments

Kalaschnik Over a year ago

Very nice. However, it is skipping the case if information is there, as in Tuesday, but no german property is there. Since it should be pushed in a new prop "unknown" ... as in the example resultObj

Keith Over a year ago

You could just do an array.filter for this, and add to your result. Updated!!

Kalaschnik Over a year ago

Just figured out, that this approach fails, unfortunately, when there are more german props for an existing prop, e.g. Monday: jsbin.com/radowuqaca/edit?js,console

Keith Over a year ago

@Kalaschnik Oh, yes. Updated!! Just needed filter again instead of find.

Nurbol Alpysbayev · Accepted Answer · 2018-12-03 15:26:55Z

2

Something like this (click the button "Run code snippet"):

const arrObj = [
{
    "day": {
    "string": "Monday",
    "Number": 1
    },
    "description": {
    "type": "string",
    "value": "The first day of the week"
    },
    "german": {
    "type": "string",
    "value": "Montag"
    }
},
{
    "day": {
    "string": "Tuesday",
    "Number": 2
    },
    "description": {
    "type": "string",
    "value": "The second day of the week"
    }
},
{
    "day": {
    "string": "Wednesday",
    "Number": 3
    },
    "description": {
    "type": "string",
    "value": "The third day of the week"
    },
    "german": {
    "type": "string",
    "value": "Mittwoch"
    }
}
];


const germanDays = ["Montag", "Dienstag", "Mittwoch", "Donnerstag"];

const resultObj = {}

for (const item of arrObj) {
    if (item.german && item.german.value && germanDays.includes(item.german.value)) {
        addVal(item.german.value, item)
    } else {
        addVal('unknown', item)
    }
}

// helper func
function addVal(key, val) {
    if(!resultObj[key]) {
        resultObj[key] = []
    }
    resultObj[key].push(val)
}

console.log(resultObj)

edited Dec 3, 2018 at 15:26

answered Dec 3, 2018 at 14:08

Nurbol Alpysbayev

22.4k6 gold badges68 silver badges100 bronze badges

2 Comments

Kalaschnik Over a year ago

Very nice. However, it is skipping the case if information is there, as in Tuesday, but no german property is there. Since it should be pushed in a new prop "unknown", as in the example resultObj

Nurbol Alpysbayev Over a year ago

Although, Stackoverflow isn't for writing the whole code for you (but to give you a direction), fixed it and now it works according to your criteria.

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