reduce method in javascript mdn code example

Example 1: javascript reduce sum

arrSum = function(arr){  return arr.reduce(function(a,b){    return a + b  }, 0);}

Example 2: reduce javascript

let sum = arr.reduce((curr, index) => curr + index);

Example 3: .reduce mdn

arr.reduce(callback( accumulator, currentValue[, index[, array]] )[, initialValue])