In Javascript, array.reduce()
is a higher-order function that takes in a function as an argument and and repeatedly applies a binary operation to the list of values to reduce the list of values in the array to a single value.
Array.prototype.reduce(callbackfn, initialValue)
callbackfn
: The callback function that is executed on each of the values in the array. callbackfn
takes in 4 additional arguments:
accumulator
: The accumulator
holds the accumulated result of applying the callbackfn
to the list. accumulator
is initialized with initial Value
if an initial value is supplied.current value
: The current value being processed in the array.index
(optional): The index of the current value being processed. index
starts from $0$ if the initial Value
is provided; otherwise, it starts from $1$.array
(optional): The array on which reduce()
is called upon.initalValue
(optional): initialValue
is used as the first value for the reduce()
operation. If it is not supplied, then the first value at index $0$ is used as the accumulator
.
reduce()
returns the accumulated result of applying the function on each of the elements in the array.
const myNumbers = [-1, 9, -10, 4, 99] const sum = myNumbers.reduce((acc, curr) => acc + curr) console.log("Sum =", sum) const sumAllPositive = myNumbers.reduce((acc, curr, index) => { if (curr > 0){ return acc + curr; } else{ return acc; } }, 0) console.log("Sum of all positive elements =", sumAllPositive)
The code above demonstrates the use of the reduce
function. Here, we use the reduce
function to first calculate the sum of all the elements in the array. Then, we proceed to calculate the sum of only the positive integers in the array.
In the first case, we call reduce
without supplying the initalValue
. The reduce()
function uses the first element as the accumulator
value, sums up all the elements one by one, and returns the result.
In the second case, we need to supply the accumulator
value because otherwise, the reduce()
function will use the value at the $0$ index as the accumulator, which in this case is a negative value and should not be counted in the sum of all positive numbers.
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