Reducing Javascript array into a map and removing duplicate values

Question

I am sure there is a clean way to do this, but I have no idea how to do it. I want to pluck a column out such that I am only returning the first occurrence of a value, but I want to keep the key that went with it.

I have a dataset that I want reduced. I want to pluck out the 'precip'.

Say I have this:

[
  "2019-01-01" => {"temp" : "cold", "season" : "winter", "precip" : "snow"},
  "2019-02-01" => {"temp" : "cold", "season" : "winter", "precip" : "none"},
  "2019-03-01" => {"temp" : "mild", "season" : "spring", "precip" : "rain"},
  "2019-04-01" => {"temp" : "mild", "season" : "spring", "precip" : "none"},
  "2019-05-01" => {"temp" : "warm", "season" : "spring", "precip" : "rain"},
  "2019-06-01" => {"temp" : "warm", "season" : "summer", "precip" : "hail"},
  "2019-07-01" => {"temp" : "hot", "season" : "summer", "precip" : "none"}
]

I would like to end up with this:

[
  "2019-01-01" => "snow",
  "2019-02-01" => "none",
  "2019-03-01" => "rain",
  "2019-06-01" => "hail"
]

I would think that Array.map has something to do with this, but I don't know how to return the key/value pair instead of just a value (i.e. map(function(d) { return d.precip }) )

What is the smooth way to do this?

Thanks in advance.

Note that it's hard to tell exactly what you are working with since the syntax is invalid. Now have 2 answers both making different assumptions — charlietfl
– charlietfl, Commented Jul 30, 2019 at 17:57
I apologize. In the title, I was referring to a Map (object). In the text, I was referencing the array map function. — jasonmclose
– jasonmclose, Commented Jul 30, 2019 at 18:04

Nina Scholz · Accepted Answer · 2019-07-31 13:05:12Z

3

You could create a Map and take only the first item with the same key.

function getFirstPrecip(data) {
    return Object.assign({}, ...Array.from(
        data.reduce((m, o) => {
            var [[k, { precip }]] = Object.entries(o);
            return m.has(precip) ? m : m.set(precip, k);
        }, new Map),
        ([k, v]) => ({ [v]: k })
    ));
}

var data = [{ "2019-01-01": { temp: "cold", season: "winter", precip: "snow" } }, { "2019-02-01": { temp: "cold", season: "winter", precip: "none" } }, { "2019-03-01": { temp: "mild", season: "spring", precip: "rain" } }, { "2019-04-01": { temp: "mild", season: "spring", precip: "none" } }, { "2019-05-01": { temp: "warm", season: "spring", precip: "rain" } }, { "2019-06-01": { temp: "warm", season: "summer", precip: "hail" } }, { "2019-07-01": { temp: "hot", season: "summer", precip: "none" } }];

console.log(getFirstPrecip(data));

edited Jul 31, 2019 at 13:05

answered Jul 30, 2019 at 17:58

Nina Scholz

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1 Comment

jasonmclose Over a year ago

So I'm trying to get this to work with my code, and am having trouble. I noticed you have a comma after the var data declaration, and then go straight into the code. How would you change it if this code were in a function, and data was an array passed as an argument?

Niche · Accepted Answer · 2019-07-30 18:13:00Z

0

You can iterate the keys for the main object, and create a new object that will use those keys and assign the precip attribute value as value for the new key=>value pair:

var dates = {
  "2019-01-01" : {"temp" : "cold", "season" : "winter", "precip" : "snow"},
  "2019-02-01" : {"temp" : "cold", "season" : "winter", "precip" : "none"},
  "2019-03-01" : {"temp" : "mild", "season" : "spring", "precip" : "rain"},
  "2019-04-01" : {"temp" : "mild", "season" : "spring", "precip" : "none"},
  "2019-05-01" : {"temp" : "warm", "season" : "spring", "precip" : "rain"},
  "2019-06-01" : {"temp" : "warm", "season" : "summer", "precip" : "hail"},
  "2019-07-01" : {"temp" : "hot", "season" : "summer", "precip" : "none"}
};
var result = Object.keys(dates).reduce((k,d) => {return [...k, {[d]:dates[d].precip}]}, [])
console.log(result)

edited Jul 30, 2019 at 18:13

answered Jul 30, 2019 at 17:57

Niche

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3 Comments

jasonmclose Over a year ago

I have a question that makes this a bit harder. What if the date was inside the key-value pairs (say "date":"2019-07-01"), and I wanted the result to be the same? So how would I pull the date out of the key-value pair and make it the new key for the returned dataset?

Niche Over a year ago

You'd just have to replace the {[d]:dates[d].precip}, where d is the date key, for {[dates[d].date]:dates[d].precip} to use the new key extracted from another value in the object.

Niche Over a year ago

You can try the above code by replaceing [d] for [dates[d].temp] to get a key:value pair from the temp and precip values in your object.

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