Try the following to search text inside your input file:
var file = event.target.files[0];
let reader = new FileReader();
reader.onload = (e: any) => {
$scope.fileContent = Reader.result;
$scope.fileName = document.getElementById("fileinput").files[0].name;
$scope.fileSize = document.getElementById("fileinput").files[0].size;
if(data.includes('Search String')){
console.log(data)
}
this.content = e.target.result;
};
reader.readAsText(file);