Edit in JSFiddle

var displayString = "";

var arr = [0, 1, 2, 3, 4];

displayString += "not use initialValue: ";
arr.reduce(function(acc, idx) {
	displayString += idx + " ";
});
$('#display').text(displayString);

displayString = "use initialValue: ";
arr.reduce(function(acc, idx) {
	displayString += idx + " ";
}, 0);
$('#display2').text(displayString);

<div id="display"></div>
<div id="display2"></div>