在JavaScript中使用Map Reduce

Question

I'm trying to use mapreduce to calculate monthly sales and graph with chart.js later.

Suppose I have a JSON response with fields, amount and date.

For my map function, I took out the month:

 month = _.map([items[i].transactionDate], function(date) { 
     return {
         lables: items[i].transactionDate.slice(5,7)
     }; 
});

and for my reduce function ... well I didn't know what to do here.

  sum = _.reduce([items[i].amt], function(memo, num) {
      return memo + items[i].amt
  });

I realized that this will eventually calculate total sum not per month. Problem is that I don't know how to relate the two functions properly.

Edit: Per request, my JSON :

 "items": [
 {
"_id": "56d1cf3704aee3c68d89cc09",
"amt": 5,
"transactionDate": "2016-02-27T16:30:47.561Z",
 }
]

and what I'm trying to get out of this is sales per month on a graph. so far I've been able to project months and total sale but not sales per a specific month.

Answer 1

map and reduce are functions of Array that iterate over the array for you

map returns a new array with the results of the mapping function for each element

var months = items.map ( item => item.transactionDate.slice(5,7) )

and reduce applies the reducing function to each element and an accumulator.

var sum = items.reduce( (accum, item) => accum + item.amt , 0);

Answer 2

I think, simply make one loop with adding new variable

let result= {};

_.forEach(items, (item) => {
    // get month
    let month = item.transactionDate.slice(5,7);
    // get amount
    let amount = item.amt;

    // check if we have this month in finaly object
    if(month in finaly) {
       // added amount
       result[month] += amount;
    } else {
       // if this month doesn't exist added with inital value
       result[month ] = amount;
    }
});

When you can get all amount of certain month or get sum of all months

let allSum = _.reduce(result, (sum, amount) => sum += amount);
let amountInCertainMonth = result["01"];

Answer 3

I assume items[i] is an object that has .transactionsDate and .amt as two arrays with corresponding indices. If so, you can get sales per month and total sales within one reduce function by starting with { totalSales, months } .

var chartData = items[i].transactionDate.reduce(function(total, month, index) {
    var monthlyData = {
        label: month.slice(5, 7),
        sales: items[i].amt[index]
    };
    total.months.push(monthlyData);
    total.totalSales += item.amt;
    return total;
}, { 
    totalSales: 0, months: []
});

// get total sales
var totalSales = chartData.totalSales;
// get sales for May starting at index 0
var salesForMay = chartdata.months[4];

Let me know if this is what you were looking for.

Answer 4

I know this is old, but here is how to use reduce as a group by

  var results = items .map(function(data){ return {"month": data.transactionDate.slice(0,7), "amt": data.amt}; }) .reduce(function(amounts, data){ if (!amounts.hasOwnProperty(data.month)) amounts[data.month] = data.amt; else amounts[data.month] = amounts[data.month] + data.amt; return amounts; }, {});