Creating key-value pairs with js reduce() and creating an array as the value

Question

I am having a problem with javascript's reduce() function; I have to put arrays as the value. I can sucessfully create an array but can't add new values to it.

There's an array with words and I have to create a 'map' whose keys are the first letters of the words and the values are the words which start with the said letter.

arr = ["Here", "is", "a", "sentence", "with", "a", "lot", "of", "words"];

The expected output should be this:

{ h: [ "here" ],
  i: [ "is" ],
  a: [ "a", "a" ],
  s: [ "sentence", ],
  w: [ "with", "words" ],
  l: [ "lot" ],
  o: [ "of" ]
}

This is my solution to the problem but it overwrites existing values.

function my_func (param)
{
   return param.reduce ( (returnObj, arr) => {
    returnObj[arr.charAt(0)] = new Array(push(arr));
    return returnObj;
  } , {})

}

I tried this but it didn't work because it couldn't deduce the type for valueOf() and it yielded an error.


function my_func (param)
{
   return param.reduce ( (returnObj, arr) => {
    returnObj[arr.charAt(0)] = (new Array(returnObj[arr.charAt(0)].valueOf().push(arr)));
    return returnObj;
  } , {})

}

I understand your question, but if you are using lodash library, you can use groupBy function like : _.groupBy( ["Here", "is", "a", "sentence", "with", "a", "lot", "of", "words"], v => v.charAt(0).toLowerCase() ); — Uday Hiwarale
– Uday Hiwarale, Commented Mar 31, 2019 at 12:54

adiga · Accepted Answer · 2019-03-31 13:59:56Z

1

You are overwriting the accumulator object's property every time. Instead, check if an item with the character has already been added using the || operator and create a new array if it doesn't exist yet.

let array = ["Here", "is", "a", "sentence", "with", "a", "lot", "of", "words"]

function my_func(param) {
  return param.reduce((acc, str) => {
    let char = str.charAt(0).toLowerCase();
    acc[char] = acc[char] || [];
    acc[char].push(str.toLowerCase());
    return acc;
  }, {})
}

console.log(my_func(array))

edited Mar 31, 2019 at 13:59

answered Mar 31, 2019 at 13:03

adiga

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1 Comment

lankovova Over a year ago

You forgot to lower case the word itself, before pushing it.

lankovova · Accepted Answer · 2019-03-31 13:02:42Z

0

Check out my solution below. Hope this helps!

const arr = ["Here", "is", "a", "sentence", "with", "a", "lot", "of", "words"];

const getWordsDict = (array) => array.reduce(
  (acc, word) => {
    const lowerCasedWord = word.toLowerCase()
    const wordIndex = lowerCasedWord.charAt(0)

    return {
      ...acc,
      [wordIndex]: [
        ...(acc[wordIndex] || []),
        lowerCasedWord,
      ],
    }
  }, 
  {}
)

console.log( getWordsDict(arr) )

answered Mar 31, 2019 at 13:02

lankovova

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irek · Accepted Answer · 2019-03-31 13:11:43Z

0

param.reduce((acc, el) => {
  const key = el[0] // use `el[0].toLowerCase()` for case-insensitive 
  if (acc.hasOwnProperty(key)) acc[key].push(el)
  else acc[key] = [el]
  return acc
}, {})

answered Mar 31, 2019 at 13:11

irek

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Comments

Uday Hiwarale · Accepted Answer · 2019-03-31 13:28:31Z

0

var result = ["Here", "is", "a", "sentence", "with", "a", "lot", "of", "words"].reduce(function(map, value) {

  var groupKey = value.charAt(0).toLowerCase();
  var newValue = value.toLowerCase();

  return map[groupKey] = map.hasOwnProperty(groupKey) ? map[groupKey].concat(newValue) : [newValue], map;
}, {});

console.log( result );

edited Mar 31, 2019 at 13:28

answered Mar 31, 2019 at 13:03

Uday Hiwarale

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1 Comment

lankovova Over a year ago

You forgot to lower case the word itself, before pushing it.

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