Convert an array of two-entry tuples into an object which maps each key to an array of values

Question

Given an array of two-entry tuples, for example:

const arr = [
    ['key1', 'value1'],
    ['key2', 'value2'],
    ['key3', 'value3'],
    ['key2', 'value4'],
    ['key3', 'value5'],
    ['key3', 'value6']
];

I'm looking for neat way to get:

const obj = {
    key1: ['value1'],
    key2: ['value2', 'value4'],
    key3: ['value3', 'value5', 'value6']
};

So far I've got:

const obj = Object.assign({}, ...arr.map(x => ({[x[0]]: x[1]})));

Which gives me:

const obj = {
    key1: 'value1',
    key2: 'value4',
    key3: 'value6'
};

I feel that I can somehow use reduce in order to obtain the desired output.

I am using NodeJS (not a browser).

phi-rakib · Accepted Answer · 2020-12-22 12:48:35Z

3

You could use Map Object.

const arr = [
  ['key1', 'value1'],
  ['key2', 'value2'],
  ['key3', 'value3'],
  ['key2', 'value4'],
  ['key3', 'value5'],
  ['key3', 'value6'],
];

const map = new Map();
arr.forEach(([x, y]) => map.set(x, map.has(x) ? [...map.get(x), y] : [y]));
const ret = Object.fromEntries(map.entries());
console.log(ret);

answered Dec 22, 2020 at 12:48

phi-rakib

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Comments

Nenad Vracar · Accepted Answer · 2020-12-22 12:28:24Z

1

Yes you could do this with reduce method where you use an object as a accumulator value.

const arr = [["key1","value1"],["key2","value2"],["key3","value3"],["key2","value4"],["key3","value5"],["key3","value6"]]

const result = arr.reduce((r, [k, v]) => {
  if (!r[k]) r[k] = [v]
  else r[k].push(v)
  return r
}, {})

console.log(result)

answered Dec 22, 2020 at 12:28

Nenad Vracar

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Comments

Adam Azad · Accepted Answer · 2020-12-22 12:28:37Z

1

Reduce is a perfect use case here

const arr = [
    ['key1', 'value1'],
    ['key2', 'value2'],
    ['key3', 'value3'],
    ['key2', 'value4'],
    ['key3', 'value5'],
    ['key3', 'value6']
];

const res = arr.reduce((acc, [key, value]) => {
  // Create the key:value and append new value
  acc[key] = acc[key] || [];
  acc[key] = [...acc[key], value];
  return acc;
}, {});

console.log(res)

answered Dec 22, 2020 at 12:28

Adam Azad

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2 Comments

goodvibration Over a year ago

So const res = arr.reduce((acc, [key, value]) => ({[...(acc[key] || []), value]}), {});?

Adam Azad Over a year ago

I would avoid nonreadable code. It is might be cool to reduce 6 lines of code to 1, but it is not maintainable. :)

Awais Mughal · Accepted Answer · 2022-12-26 10:57:01Z

0

Here's how I'm using this in my React/TS project to create select options!

export const mapToSelectOptionsFormTupleOfTwo = (
    tupleArray: ["string", "string"][]
) => {
    const selectOptions = new Array();
    tupleArray?.forEach(([x, y]) =>
        selectOptions.push({ value: lowercaseFirstLetter(x), label: y })
    );

    return selectOptions;
};

answered Dec 26, 2022 at 10:57

Awais Mughal

4194 silver badges12 bronze badges

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