what is accumulator in reduce javascript code example

Example 1: array reduce

arr.reduce(callback( accumulator, currentValue[, index[, array]] ) {
  // return result from executing something for accumulator or currentValue
}[, initialValue]);

Example 2: reduce method javascript

// Reduce() method executes a callback function that is passed in
// on each element of the array, resulting in single output value.
const array1 = [1, 2, 3, 4];
const callback = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(callback));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(callback, 5));
// expected output: 15 because the initial value is 5.

// This is how Reduce works.
// This is a myReduce method which takes a callback and an optional argument 
// of a default accumulator. If myReduce only receives one argument, then 
// myReduce will use the first element as the accumulator.

Array.prototype.myReduce = function(callback, acc) {
    if (!acc) {
        acc = this.shift();
    }
    this.forEach(function(element) {
        acc = callback(acc, element)
    })
    return acc;
}