Question 52554 in Javascript Question posted in
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Javascript create dictionary from another dictionary

If I have a dict like below:

const d = {"S1": [2, 3, 4], "S2": [5, 6, 7, 8]}

I want to create another dictionary that can get the len or the 1st element as value with the same key, like:

const n = {"S1": 3, "S2": 4} # len of the value array

I try to use a map like below:

Object.entries(d).map(([k, v]) => console.log(k, v))

It prints the key-value pair but how do I use that to create a new dictionary?

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2

Answers


  1. 0

    you have taken a good start in the right direction.
    Object.fromEntries can be used to convert the key value pair array back to an object with the Object.entries + map you have started with

    const d = {"S1": [2, 3, 4], "S2": [5, 6, 7, 8]}

const res = Object.fromEntries(Object.entries(d).map(([k,v]) => [k,v.length]))

console.log(res)

    as an alternative you could achieve the same thing with Object.entries + reduce

    const d = {"S1": [2, 3, 4], "S2": [5, 6, 7, 8]}

const res = Object.entries(d).reduce((acc,[k,v]) => ({...acc,[k]:v.length}),{})

console.log(res)
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  2. 0

    See inline comments:

    function mapLengths(obj) {
  // Convert entries back to object:
  return Object.fromEntries(
    // Convert "d" to entries (tuples) and map:
    Object.entries(obj).map(
      // Return a tuple with the key and value of the "length" property:
      ([k, v]) => [k, v?.length],
    ),
  );
}

const d = { "S1": [2, 3, 4], "S2": [5, 6, 7, 8] };
const n = mapLengths(d);
console.log(n); // { S1: 3, S2: 4 }

// If there is a value without a "length" property,
// the optional chaining operator will prevent an invalid access exception
// and just return "undefined":

const d2 = {
  "S0": null,
  "S1": [2, 3, 4],
  "S2": [5, 6, 7, 8],
  "S3": {},
  "S4": "hello world",
};

const n2 = mapLengths(d2);
console.log(n2); // { S0: undefined, S1: 3, S2: 4, S3: undefined, S4: 11 }
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