JavaScript: Reduce an array to nested objects

Question

So suppose my array looks like this:

let langArr = [
     ["python", "blue"]
    ,["python", "blue"]
    ,["c++", "red"]
    ,["java", "yellow"]
    ,["javascript", "lime"]
    ,["shell", "green"]
    ,["c++", "red"]
];

what I want is something like this:

{
  python: {
    count: 2
    color: "blue"
  }
  c++: {
    count: 2
    color: "red"
  }
  java: {
    count: 1
    color: "yellow"
  }
  and so on...
}

I tried reduce method like this:

let langCount = langArr.reduce((lang, [name, color]) => {
      lang[name] = (lang[name] || 0) + 1;
      lang[color] = 'color';
      return lang;
    }, {});
    console.log(langCount);

but I get this output:

{
  python: 2
  blue: "color"
  c++: 2
  red: "color"
  java: 1
  yellow: "color"
  and so on...
}

What have you tried to modify the .reduce() snippet so it produces the output you're expecting? — Andreas
– Andreas, Commented Mar 9, 2020 at 15:29
You lack creation of the sub-objects, and one indirection in the reduce code. Most notably, the word count never occurs in your code, which should make the primary issue obvious - if you had a place to put that, you'd (probably) have the desired structure. — ASDFGerte
– ASDFGerte, Commented Mar 9, 2020 at 15:30
@Andreas that is what I tried, I don't know how to get them into separate objects — m00
– m00, Commented Mar 9, 2020 at 15:32

Nina Scholz · Accepted Answer · 2020-03-09 15:46:54Z

1

You need an object for each language.

This approach takes an object as default value if lang[name] is falsy, like undefined.

The pattern

variable = variable || value;

works with a logical OR ||:

if variable has a truthy value, take this value,
if variable has a falsy value, take value instead.

let langArr = [["python", "blue"], ["python", "blue"], ["c++", "red"], ["java", "yellow"], ["javascript", "lime"], ["shell", "green"], ["c++", "red"]],
    langCount = langArr.reduce((lang, [name, color]) => {
        lang[name] = lang[name] || { count: 0, color };
        lang[name].count++;
        return lang;
    }, {});

console.log(langCount);

edited Mar 9, 2020 at 15:46

answered Mar 9, 2020 at 15:33

Nina Scholz

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2 Comments

Andreas Over a year ago

This is just another "group by" answer (with the little twist, that it's not an array with all options but a number for the count). This is now already enough for a new answer instead of a dupe-vote?

m00 Over a year ago

Thank you so much! can you also please explain what this line does lang[name] = lang[name] || { count: 0, color };

Dimitar Dimitrov · Accepted Answer · 2020-03-09 15:54:45Z

0

You can use this:

array.reduce((acc, current) => {
        if(!acc.hasOwnProperty(current[0])){
            acc[current[0]] = {count: 0, color: current[1]};
        }
        acc[current[0]].count += 1;
        return acc;
    }, {});

answered Mar 9, 2020 at 15:54

Dimitar Dimitrov

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