I researched about this a lot and finally I could come up with a solution to this.

public exportAsExcelFile(json: Array<object>, excelFileName: string): void {

var worksheet: XLSX.WorkSheet = XLSX.utils.aoa_to_sheet([

  [`${excelFileName}`]]); // message to display

worksheet['!merges'] = [{ s: { r: 0, c: 0 }, e: { r: 0, c: 3 } }]; //for merging columns. s : start, e: end, c: column, r: row

XLSX.utils.sheet_add_json(worksheet, json, { origin: A2 }); //origin for json data

const workbook: XLSX.WorkBook = { Sheets: { 'data': worksheet }, SheetNames: ['data'] };

const excelBuffer: any = XLSX.write(workbook, { bookType: 'xlsx', type: 'array' });

var range = XLSX.utils.decode_range(worksheet['!ref']);

for (var C = range.s.r; C <= range.e.r; ++C) {

  var address = XLSX.utils.encode_col(C) + 1; 

  if (!worksheet[address]) continue;

  worksheet[address].v = worksheet[address].v.charAt(0).toUpperCase() + worksheet[address].v.substr(1).toLowerCase();

}

}