Creating arrays in JavaScript by comparing values in another array

Question

In JavaScript, I'm trying to create arrays based on the values of another array and I'm having difficultly.

I've got an array of dates in string format (dates) e.g.

["30/09/2015", "31/10/2015", "30/11/2015", "31/12/2015"]

I've got an Object to represent multiple bank accounts (balancesSortByAccId) e.g.

Cash - (Array size: 3)
id: 1, date: "30/09/2015", balance: 30
id: 2, date: "31/10/2015", balance: 50
id: 3, date: "30/11/2015", balance: 100

Natwest - (Array size: 2)
id: 4, date: "30/11/2015", balance: 400
id: 5, date: "31/12/2015", balance: 200

Whilst looping through all the accounts in balancesSortByAccId, I want to be able to create an array for the balance at each date in the dates array i.e.

[30, 50, 100, null]
[null, null, 400, 200]

How could I achieve this?

UPDATE: jsfiddle code - https://jsfiddle.net/gx8bLehb/

Please format the strings in your code as actual strings (e.g. with quotes around them). — jfriend00
– jfriend00, Commented Jan 29, 2016 at 18:13
Hey, did any of the answers solve your problem? If yes, please accept it. — Johannes Jander
– Johannes Jander, Commented Jan 30, 2016 at 8:15
@JohannesJander I am still attempting to resolve this. I understand that my original question/jsfiddle code isn't clear and so I was wondering whether I should delete this question and re-post again or just modify the OP? — Bhav
– Bhav, Commented Jan 30, 2016 at 10:15

Johannes Jander · Accepted Answer · 2016-01-29 20:34:51Z

3

The easiest way would be to transform your cash and natwest arrays into a hash sorted by date, something like balancesByDate:

    var balancesByDate = _.groupBy(cash, function(entry) {return entry.date});

Then use an array map() function, e.g. from lodash to iterate the dates array and for each date look up the account line in the balancesByDate hash. From that line, return the balance property in the map function.

dates.forEach(function(date){
  if (balancesByDate[date]) {
    results.push(_.map(balancesByDate[date], function(line){
       return line.balance;
    }));
  } else {
    results.push(null);
  }
});

However, you need to be aware that your dataset most likely could contain duplicate balances for a day, you should plan for that (my code returns an array for each day).

https://jsfiddle.net/hdpuuc5d/1/

edited Jan 29, 2016 at 20:34

answered Jan 29, 2016 at 18:03

Johannes Jander

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4 Comments

Heretic Monkey Over a year ago

Or, you know, the built-in Array.prototype.map instead of importing an extra library... ;)

Johannes Jander Over a year ago

@MikeMcCaughan: yes, depending on the browser version OP needs to support.

Bhav Over a year ago

@JohannesJander How can I transform balancesSortByAccId into a hash as you mentioned? I've added a link to jsfiddle with sample data/my code in the OP.

Johannes Jander Over a year ago

I have amended my answer in response to the fiddle. Unfortunately, in the fiddle, the data structure is somewhat different than in the question, that's not really nice. Also there were bugs and undeclared variables in the fiddle - please if you want help from people, do check and correct your stuff. Anyhow, here's a working example: jsfiddle.net/hdpuuc5d/1 - please be aware that you can have more than one balance result for a given date - therefore the results-array contains an array for each day.

Nina Scholz · Accepted Answer · 2016-01-29 18:52:59Z

A solution in plain javascript with a helper object for the dates:

var dates = ["30/09/2015", "31/10/2015", "30/11/2015", "31/12/2015"],
    datesObj = dates.reduce(function (r, a, i) { r[a] = i; return r; }, {}),
    balances = {
        Cash: [
            { id: 1, date: "30/09/2015", balance: 30 },
            { id: 2, date: "31/10/2015", balance: 50 },
            { id: 3, date: "30/11/2015", balance: 100 }
        ],
        Natwest: [
            { id: 4, date: "30/11/2015", balance: 400 },
            { id: 5, date: "31/12/2015", balance: 200 }
        ]
    },
    result = {};

Object.keys(balances).forEach(function (k) {
    result[k] = Array.apply(null, { length: dates.length }).map(function () { return null; });
    balances[k].forEach(function (a) {
        result[k][datesObj[a.date]] = a.balance;
    });
});

document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

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