The reduce() method invokes a function
on each element of the array.
The value returned by one invocation
is passed as argument to the next.
In the above example:
-
The function calculateSum passed as the
first argument to reduce()
is invoked on all elements of numbers.
The second argument of reduce() is usually
referred to as the initial value.
-
When calculateSum is first invoked,
the first argument passed to calculateSum
is the initial value - 0.
The second argument passed to calculateSum
is the first element of numbers, i.e., 1.
-
In subsequent invocations of calculateSum,
the first argument will be
the value returned by the previous invocation.
The second argument will be
subsequent elements of the numbers array.
-
The value returned by the
last invocation will be the value
returned by the reduce() method.
The function passed as the first argument
to the reduce() method can accept four arguments:
-
Stores the value returned
from the previous function call.
-
The value of the current element
in the array.
-
The index of the current element
in the array.
-
The array reduce is invoked on.
In the example given above,
the initial value of reduce is omitted.
By default reduce takes the value of
the first element of the array it is invoked on
as the initial value.