map and reduce an array at the same time in javascript

Question

I have an array of ~1800 object represents games played in a league. I need a new array that has an object for each team, and will include 4 new fields (wins, losses, ties, and points). Here is a sample of the array I am working with:

[
  {
    "homeGoals": 2,
    "gameId": "12221",
    "homeTeam": {
      "id": "aasfdsf1",
      "teamName": "Team 1"
    },
    "awayTeam": {
      "id": "aasfdsf2",
      "teamName": "Team 2"
    },
    "id": "ggaew1",
    "awayGoals": 4
  },
  {
    "homeGoals": 5,
    "gameId": "12222",
    "homeTeam": {
      "id": "aasfdsf1",
      "teamName": "Team 1"
    },
    "awayTeam": {
      "id": "aasfdsf3",
      "teamName": "Team 3"
    },
    "id": "ggaew2",
    "awayGoals": 1
  },
  {
    "homeGoals": 4,
    "gameId": "12223",
    "homeTeam": {
      "id": "aasfdsf2",
      "teamName": "Team 2"
    },
    "awayTeam": {
      "id": "aasfdsf3",
      "teamName": "Team 3"
    },
    "id": "ggaew3",
    "awayGoals": 4
  },
  {
    "homeGoals": null,
    "gameId": "12223",
    "homeTeam": {
      "id": "aasfdsf2",
      "teamName": "Team 2"
    },
    "awayTeam": {
      "id": "aasfdsf3",
      "teamName": "Team 3"
    },
    "id": "ggaew4",
    "awayGoals": null
  }
]

And here is an example of what I need the result to look like:

 [
  {
    "id": "aasfdsf1",
    "name": "Team 1",
    "wins": 1,
    "losses": 1,
    "ties": 0,
    "points": 2 
  },
  {
    "id": "aasfdsf2",
    "name": "Team 2",
    "wins": 1,
    "losses": 0,
    "ties": 1,
    "points": 3 
  },
  {
    "id": "aasfdsf3",
    "name": "Team 3",
    "wins": 0,
    "losses": 1,
    "ties": 1,
    "points": 1 
  }
]

Some games have not been played, so the homeGoals and awayGoals fields will be null.

So far I have a list of unique teams, only where the games have been completed:

const completedGames = games.filter(x => x.homeGoals !== null)
const homeTeams = [...new Set(completedGames.map(x => x['homeTeam']))];
const awayTeams = [...new Set(completedGames.map(x => x['awayTeam']))];
const teams = [...new Set([...homeTeams, ...awayTeams])]

I know I need to do some sort of reduce function, but am having trouble figuring it. I am pretty sure the step I just did before would be irrelevant if I had a proper map reduce function. Any help would be greatly appreciated !

georg · Accepted Answer · 2017-09-22 16:07:11Z

3

This can be expressed in a simpler way with flatMap. It's not built-in in JS, but easy to implement:

let flatMap = (a, fn) => [].concat(...a.map(fn));

Now, on the map step, you can emit two "result" object per game (or no results at all if the game is incomplete):

results = flatMap(data, g => {

    if (g.homeGoals === null || g.awayGoals === null)
        return [];

    if (g.homeGoals > g.awayGoals)
        return [
            {id: g.homeTeam.id, r: 'win'},
            {id: g.awayTeam.id, r: 'loss'},
        ];

    if (g.homeGoals < g.awayGoals)
        return [
            {id: g.homeTeam.id, r: 'loss'},
            {id: g.awayTeam.id, r: 'win'},
        ];

    if (g.homeGoals === g.awayGoals)
        return [
            {id: g.homeTeam.id, r: 'tie'},
            {id: g.awayTeam.id, r: 'tie'},
        ];
});

This creates an array like

{ id: 'aasfdsf1', r: 'loss' },
{ id: 'aasfdsf2', r: 'win' },
{ id: 'aasfdsf1', r: 'win' }, etc

which is easy to reduce:

summary = results.reduce((m, {id, r}) => {
    let e = m[id] || {};
    e[r] = (e[r] || 0) + 1;
    return Object.assign(m, {[id]: e})
}, {});

You can also make is less verbose by encoding wins, losses and ties by 1, -1, 0 respectively, in which case the mapper becomes:

results = flatMap(
    data.filter(g => g.homeGoals !== null),
    g => {
        let d = g.homeGoals - g.awayGoals;
        return [
            {id: g.homeTeam.id, r: Math.sign(+d)},
            {id: g.awayTeam.id, r: Math.sign(-d)},
        ]
    });

edited Sep 22, 2017 at 16:07

answered Sep 22, 2017 at 15:46

georg

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2 Comments

0xPingo Over a year ago

Thank you for the help !

0xPingo Over a year ago

The summary function is returning an object of objects, how do i get that to be an array? I tried changing the {} at the end of the function to [], but that returned a blank array.

jmtalarn · Accepted Answer · 2017-09-22 15:39:03Z

I think you are looking to something like this:

const hashMapTeams = games.filter(x => x.homeGoals !== null)
.reduce((res, match)=>{
   /* do the calculations here */
   /* put the values on the res object, using res as a HashMap*/
    res["/*the home team id*/"].id = /*id value*/
    res["/*the home team id*/"].name = /*name value*/
    res["/*the home team id*/"].wins= /* the right value */;
    res["/*the home team id*/"].losses= /* the right value */;
    res["/*the home team id*/"].ties= /* the right value */;
    res["/*the home team id*/"].points= /* the right value */;

    res["/*the away team id*/"].id = /*id value*/
    res["/*the away team id*/"].name = /*name value*/
    res["/*the away team id*/"].wins= /* the right value */;
    res["/*the away team id*/"].losses= /* the right value */;
    res["/*the away team id*/"].ties= /* the right value */;
    res["/*the away team id*/"].points= /* the right value */;
 },{});

/* This will convert again the object to an array */
const arrayTeams = Object.keys(hashMapTeams).map(function (key) { return hashMapTeams[key]; });

Andy · Accepted Answer · 2017-09-22 16:22:51Z

This gets the exactly result you are looking for:

{
  "id": "aasfdsf1",
  "name": "Team 1",
  "wins": 1,
  "losses": 1,
  "ties": 0,
  "points": 2 
},

I used tenary and brackets to show you more than one way to approach that, you can use either one.

let result = [];

your1800ArrayObj.map(data => {      
    let wins = data.wins ? data.wins : 0;
    let losses = data.losses ? data.losses : 0;
    let ties = data['ties'] || 0;
    let points = data['points'] || 0;

    if (data.homeGoals === null && data.awayGoals === null) {
        console.log('game not played')
        } else {
            if (data.homeGoals > data.awayGoals) {
                wins += 1
                points += 1
            } else if (data.homeGoals < data.awayGoals) {
                losses += 1
            } else {
                ties += 1
            }
        }

    result.push({
        id: data.id,
        name: data.homeTeam.teamName ,
        wins: wins,
        losses: losses,
        ties: ties,
        points: points
    })
})

return result
}

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