Javascript reduce for certain objects within an array of objects

Question

I'm having a little problem here with counting array of object values... The code I have below works just fine but what if I want to target only certain objects within the reduce function?

Basically, something like an SQL WHERE function, so

newcost = Number((cart.map(item => (item.cost * item.amm)).reduce((prev, next) => Number(prev) + Number(next))).toFixed(2));

would be

newcost = Number((cart.map(item => (item.cost * item.amm)).reduce((prev, next) => Number(prev) + Number(next))).toFixed(2)) WHERE item.type === "c";

You know, something similar to this. How can I achieve something like this?

Thank you.

you could put a .filter in between: x.map().filter(...).reduce(...) — georg
– georg, Commented Apr 16, 2018 at 0:42

Mark · Accepted Answer · 2018-04-16 01:04:18Z

Here is how you could do it with filter(). filter() returns an array where the function passed to it returns true. This has the effect of only retuning items of type 'c'.

var cart = [
    {type:'c', cost:20.00, amm: 10},
    {type:'d', cost:20.00, amm: 1},
    {type:'d', cost:20.00, amm: 2},
    {type:'c', cost:1.00, amm: 5},
    {type:'a', cost:20.00, amm: 7},

]
let newcost = cart.filter(i => i.type === 'c') // only c type items
                  .map(item => item.cost * item.amm)
                  .reduce((prev, next) => prev + next)
                  .toFixed(2);

console.log(newcost)

Also, you didn't ask, but the map() call is extraneous — you don't really need it and it causes an extra loop through your data (you could also just do the test in reduce() and leave out the filter() although that might start to impact readability):

var cart = [
    {type:'c', cost:20.00, amm: 10},
    {type:'d', cost:20.00, amm: 1},
    {type:'d', cost:20.00, amm: 2},
    {type:'c', cost:1.00, amm: 5},
    {type:'a', cost:20.00, amm: 7},

]
let newcost = cart.filter(i => i.type === 'c')
                  .reduce((prev, next) => prev + next.cost * next.amm, 0)
                  .toFixed(2);

console.log(newcost)

Geuis · Accepted Answer · 2018-04-16 01:09:13Z

1

Add your conditional first in the reduce function. If the element doesn’t match your conditional, just return the accumulator first without modifying it.

answered Apr 16, 2018 at 1:09

Geuis

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