sum javascript

const sum = arr => arr.reduce((a, b) => a + b, 0);

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

const arrSum = arr => arr.reduce((a,b) => a + b, 0)

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

function getArraySum(a){
    var total=0;
    for(var i in a) { 
        total += a[i];
    }
    return total;
}

var payChecks = [123,155,134, 205, 105]; 
var weeklyPay= getArraySum(payChecks); //sums up to 722

const summation = (n) => (n * (n + 1)) / 2;

 <button onclick="myFunction()">Try it</button>

<p>Sum of numbers in array: <span id="demo"></span></p>

<script>
var numbers = [15.5, 2.3, 1.1, 4.7];

function getSum(total, num) {

    return total + Math.round(num);
}

function myFunction(item) {
  document.getElementById("demo").innerHTML = numbers.reduce(getSum, 0);
}

 </script>