use of reduce in array javascript code example

Example 1: syntax of reduce in js

[1,2,3,4,5].reduce((acc, current)=>acc+current, 0)

Example 2: reduce javascript

//note idx and sourceArray are optional
const sum = array.reduce((accumulator, element[, idx[, sourceArray]]) => {
	//arbitrary example of why idx might be needed
	return accumulator + idx * 2 + element 
}, 0);