build object from another object that compares its keys to values in an array

Question

The following object is used as a starting point to populate options of a select box in the UI of an app:

const months = {
  "1": "Jan",
  "2": "Feb",
  "3": "Mar",
  "4": "Apr",
  "5": "May",
  "6": "Jun",
  "7": "Jul",
  "8": "Aug",
  "9": "Sep",
  "10": "Oct",
  "11": "Nov",
  "12": "Dec"
}

The options for the select box must be limited though based on the contents of another array. Here is an example of that array:

const existingMonths = [
  1,
  2,
  3,
  4,
  5,
  6,
  7,
  8,
  12
];

So in this example, the object of final options for the select box should be:

const availableMonths = {
  "9": "Sep",
  "10": "Oct",
  "11": "Nov"
}

I'm having difficulty figuring out how to build the availableMonths object. This does not give the desired output:

const availableMonths = Object.entries(months).filter(k => !existingMonths.includes(k));

k is an array with two entries - the key first, the value second. If you want to check if the key exists in another array, you need to just take the first element. Also, the key would be a string, while the existingMonths array contains numbers. You need to convert the two to the same type. — VLAZ
– VLAZ, Commented May 18, 2021 at 14:20

marc_s · Accepted Answer · 2021-11-03 21:31:22Z

4

You need to take the key from the array of entries and check with the numerical value of the key against the value of the array. Finally create a new object from the entries.

const
    months = { 1: "Jan", 2: "Feb", 3: "Mar", 4: "Apr", 5: "May", 6: "Jun", 7: "Jul", 8: "Aug", 9: "Sep", 10: "Oct", 11: "Nov", 12: "Dec" },
    existingMonths = [  1,  2,  3,  4,  5,  6,  7,  8,  12],
    availableMonths = Object.fromEntries(Object
        .entries(months)
        .filter(([k]) => !existingMonths.includes(+k))
    );

console.log(availableMonths);

edited Nov 3, 2021 at 21:31

marc_s

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answered May 18, 2021 at 14:23

Nina Scholz

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2 Comments

knot22 Over a year ago

What does the +k do?

ulou Over a year ago

@knot22 parsing string to number

ulou · Accepted Answer · 2021-05-21 13:22:50Z

1

const months = {
  "1": "Jan",
  "2": "Feb",
  "3": "Mar",
  "4": "Apr",
  "5": "May",
  "6": "Jun",
  "7": "Jul",
  "8": "Aug",
  "9": "Sep",
  "10": "Oct",
  "11": "Nov",
  "12": "Dec"
}

const existingMonths = [1, 2, 3, 4, 5, 6, 7, 8, 12]

const availableMonths = Object.fromEntries(
   Object.entries(months).filter(([k, v]) => !existingMonths.includes(+k))
)

console.log(availableMonths)

edited May 21, 2021 at 13:22

answered May 18, 2021 at 14:25

ulou

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Comments

prashanth Sasidharan · Accepted Answer · 2021-05-25 07:58:00Z

1

Step 1: Shallow Copy months object
Step 2: Loop throught existingMonths
Step 3: Delete shallow copied properties by comparing with existingMonths elements

let
months = { 1: "Jan", 2: "Feb", 3: "Mar", 4: "Apr", 5: "May", 6: "Jun", 7: "Jul", 8: "Aug", 9: "Sep", 10: "Oct", 11: "Nov", 12: "Dec" },
existingMonths = [  1,  2,  3,  4,  5,  6,  7,  8,  12],availableMonths = {}

Object.assign(availableMonths,months);

existingMonths.forEach((monthNumber) =>{delete availableMonths[monthNumber]})
   

console.log(availableMonths);

edited May 25, 2021 at 7:58

answered May 18, 2021 at 15:13

prashanth Sasidharan

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