function reduce(array, combine, start) {

  let current = start;

  for (let element of array) {

    current = combine(current, element);

  }

  return current;

}

​

console.log(reduce([1, 2, 3, 4], (a, b) => a + b, 0));

// → 10

const array1 = [1, 2, 3, 4];

const reducer = (accumulator, currentValue) => accumulator + currentValue;

​

// 1 + 2 + 3 + 4

console.log(array1.reduce(reducer));

// expected output: 10

​

// 5 + 1 + 2 + 3 + 4

console.log(array1.reduce(reducer, 5));

// expected output: 15

const arrayOfNumbers = [1, 2, 3, 4];

const sum = arrayOfNumbers.reduce((accumulator, currentValue) => {  

  return accumulator + currentValue;

});

console.log(sum); // 10

// define a reusable function

const calculateSum = (arr) => {

    return arr.reduce((total, current) => {

        return total + current;

    }, 0);

}

​

// try it

console.log(calculateSum([1, 2, 3, 4, 5]));

const list = [1, 2, 3, 4, 5];

console.log(list.reduce((number, nextNumber) => number + nextNumber));

// --> 15

/*

This method takes a callback that takes two parameters,

one that represents the element from the last iteration and the other is the current

element of the iteration

*/

​

let nums = [2,4,6,8,3,5]

​

let result = nums.reduce((prev,curr)=>prev+curr)

console.log(result); // 28

n JavaScript, the reduce() method is used to apply a function to each element of an array and reduce it to a single value. The reduce() method takes two parameters: a callback function and an optional initial value.

​

Here is the basic syntax of the reduce() method:

​

'array.reduce(callback, initialValue)'

​

The callback function takes four parameters: accumulator, currentValue, currentIndex, and array. The accumulator is the value returned from the previous invocation of the callback, or the initialValue if provided. The currentValue is the current element being processed, currentIndex is the index of the current element, and array is the array on which reduce() was called.

​

The reduce() method executes the callback function once for each element in the array (excluding the initial value if provided), and the return value of the callback becomes the new value of the accumulator.

​

Here is an example that demonstrates the usage of reduce() to calculate the sum of an array of numbers:

​

const numbers = [1, 2, 3, 4, 5];

​

const sum = numbers.reduce((accumulator, currentValue) => {

  return accumulator + currentValue;

}, 0);

​

console.log(sum); // Output: 15

​

​

In the example above, the reduce() method is called on the numbers array. The callback function takes two parameters, accumulator and currentValue, and simply adds them together. The initial value of the accumulator is set to 0 using the second argument of reduce(). The final result, 15, is the sum of all the numbers in the array.