[{name: '4 Arn', isLetter: false},
{name: 'Abax', isLetter: false},
{name: 'Aramex', isLetter: false},
{name: 'Booking', isLetter: false},
{name: 'Dangerous', isLetter: false},
{name: 'Manali', isLetter: false}]

[{letter: "#", isLetter: true},       // new object
{name: '4 Arn', isLetter: false},
{letter: "A", isLetter: true},        // new Object
{name: 'Abax', isLetter: false},
{name: 'Aramex', isLetter: false},
{letter: "B", isLetter: true},        // new object
{name: 'Booking', isLetter: false},
{letter: "D", isLetter: true},        // new object
{name: 'Dangerous', isLetter: false},
{letter: "M", isLetter: true},        // new object
{name: 'Manali', isLetter: false}]

var newArr = [];
list.reduce(function(prev, cur, index, originalArrray) {
    var previousCharcode = prev.name.toUpperCase().charCodeAt(0);
    currentCharCode = cur.name.toUpperCase().charCodeAt(0);

    newArr.push(prev);
    if(previousCharcode != currentCharCode) {
        newArr.splice(index, 0, {isLetter: true, letter: String.fromCharCode(currentCharCode)});
    }
    return cur;
});