Find sum of objects inside array using reduce

Question

I am trying to find the total from objects inside an array, which each object has a price and quantity, i can find the total when the array has exactly two objects, but for more than two it produces NaN.

arr = [ { quantity: 1, price: 30 },
{ quantity: 1, price: 40 },
{ quantity: 2, price: 10 },
{ quantity: 1, price: 10 } ]


const reducer = (accumulator, currentValue) => {
    var a = accumulator.quantity * accumulator.price;
    var b = currentValue.quantity * currentValue.price;
    return a + b;
}
console.log(arr.reduce(reducer));  // sum if array contains 2 objects, NaN otherwise.

accumulator won't have a quantity or price properties, since it's the result of the last return value. Since you do a + b, you'd only get a number the first time, but on the second iteration, you'll have accumulator = <previous>a + <previous>b — VLAZ
– VLAZ, Commented Oct 22, 2019 at 16:59
Check out the docs for reduce, specifically what it used as the initial accumulator value if none is provided. — Dave Newton
– Dave Newton, Commented Oct 22, 2019 at 17:00
@DaveNewton the initial is OK, in this case. But only the initial. There is a problem for anything other than two items. — VLAZ
– VLAZ, Commented Oct 22, 2019 at 17:01

Isaac Khoo · Accepted Answer · 2019-10-22 16:59:40Z

2

    let arr = [ 
    { quantity: 1, price: 30 },
    { quantity: 1, price: 40 },
    { quantity: 2, price: 10 },
    { quantity: 1, price: 10 } 
    ]

    let reducer = (acc, cur) => {
     return acc + (Number(cur.quantity) * Number(cur.price));
    };

    console.log(arr.reduce(reducer, 0));
    // 100

Your reducer function seems to be wrong. Accumulator no longer has any parameters to it, since well, it accumulates - its an integer. Also, set a initial value for your accumulator to start accumulating from, as shown in the reduce function, second parameter input

answered Oct 22, 2019 at 16:59

Isaac Khoo

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3 Comments

VLAZ Over a year ago

Why explicitly convert the numbers to numbers?

Isaac Khoo Over a year ago

Just in case somewhere in the input object, the value is stored as string

VLAZ Over a year ago

None are here, so it's a bit useless to think of this case. And what if the value is a boolean? And what if it's undefined? What if the value is a string but it's "apple"? Not really worth it solving all these potential issues. Not to mention that the * operator will do an implicit casting to number for both operands. Even if one of the values is the string "10" instead of the number 10 that will be handled. And you'll still have the same problems with non-numeric strings and other types.

DXNNX · Accepted Answer · 2019-10-22 17:01:07Z

0

arr = [ { quantity: 1, price: 30 },
{ quantity: 1, price: 40 },
{ quantity: 2, price: 10 },
{ quantity: 1, price: 10 } ]

const reducer = (accumulator, currentValue) {
    return accumulator + (currentValue.quantity * accumulator.price);
}

console.log(arr.reduce(reducer, 0 ));

answered Oct 22, 2019 at 17:01

DXNNX

111 bronze badge

Comments

Eslam Abu Hugair · Accepted Answer · 2019-10-22 17:02:26Z

-1

you can simply say

arr = [ { quantity: 1, price: 30 },
{ quantity: 1, price: 40 },
{ quantity: 2, price: 10 },
{ quantity: 1, price: 10 } ]


const total = arr.reduce((total,item)=>{
   total +=  item.quantity * item.price;
  return total
},0)

edited Oct 22, 2019 at 17:02

answered Oct 22, 2019 at 17:01

Eslam Abu Hugair

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1 Comment

VLAZ Over a year ago

not the callback doesn't return anything. The whole reduce will return undefined.

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