JavaScript reduce throwing error for more than two items

Question

I have this array:

const arr = [
  { someProp: [{ amount: 10 }]},
  { someProp: [{ amount: 12 }]},
];

and then this reduce fn:

con         


        

Answer 1

You're returning a number. The value of prev is always the prior return value from the callback, not the previous element. Thus on the third iteration you're trying to use that number as if it were an object reference.

Add a second parameter (0) to the .reduce() call, and change the function to treat prev as the running total (a simple number):

const sum = arr.reduce((prev, curr) => prev + curr.someProp[0].amount, 0);

It works as-is when there are two elements because without that second parameter, the first iteration will see element 0 as prev and element 1 as curr. That behavior works fine when you've got an array of numbers and you just want to perform a computation between them, but in this case you need that initial value explicitly there as the second argument.