JavaScript `reduce` yielding NaN when trying to sum simple floats [closed]

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let totalIn = transactions.reduce((a, b) => Number(a.amount) + Number(b.amount));

What could be reasons that this is yielding NaN? Each object in the transactions array has an amount property and each amount property is holding a string that is convertible to a float.

Please edit your question to add a example of transactions. — 0stone0
– 0stone0, Commented Jul 20, 2021 at 13:52
the second argument of reduce is also missing, which is the initial value but in the case of an object You should do it externally — DecPK
– DecPK, Commented Jul 20, 2021 at 13:53
let totalIn = transactions.reduce((acc, cur) => acc + cur.amount, 0); is probably what you want — Sleepwalker
– Sleepwalker, Commented Jul 20, 2021 at 13:55
The correct code would be let totalIn = transactions.reduce((total, item) => total += Number(item.amount), 0). You are misinterpreting the arguments of the reduce callback function and you did not initialize the accumulator. — secan
– secan, Commented Jul 20, 2021 at 13:55

Pointy · Accepted Answer · 2021-07-20 13:53:05Z

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The .reduce() method passes the accumulator value as the first parameter. After the first iteration, therefore, a will not be one of the values in your array: it will be the numeric sum of two numbers. Therefore it won't have an "amount" property, so Number(a.amount) will be NaN.

The way I personally would deal with that would be to ensure that every iteration is the same:

let totalIn = transactions.reduce((a, b) => a + Number(b.amount), 0);

By passing in a second parameter to .reduce() to be used as the initial value of the accumulator, every call to the reduce callback will be similar (a number as a and one of your array element objects as b).

answered Jul 20, 2021 at 13:53

Pointy

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