I'm having trouble understanding the reduce method. What is the reason why
const sumOfCubes = nums => nums.reduce((a,b) => a + Math.pow(b, 3))
does not function properly, while
const sumOfCubes = nums => nums.reduce((a,b) => a + Math.pow(b, 3), 0)
works as expected?
When using only one argument, array.reduce(fn) functions similarly to this:
for (let i = 1, a = array[0]; i < array.length; ++i)
a = fn(a, array[i]);
This means that in your scenario, the first value of 2 is not cubed, resulting in an incorrect sum.
Adding a second argument changes the behavior to:
for (let i = 0, a = secondParameter; i < array.length; ++i)
a = fn(a, array[i]);
By passing 0 as the second parameter, the value to add on the first iteration is correctly set to 0, leading to each value in the array being cubed.
In the context of `reduce`, it is important to specify the initial value of the accumulator, as demonstrated in the second example by passing `0` as an argument.
For more information on this topic, check out the following resource:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce
As pointed out by @VLAZ, if you do not provide an initial value, the reduce function will use the first element of the array but skip it:
InitialValue Optional A value to use as the first argument to the first call of the callback. If no initialValue is supplied, the first element in the array will be used and skipped. Calling reduce() on an empty array without an initialValue will throw a TypeError.
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