[danik.js]

function getText(url, callback) {
    var request = new XMLHttpRequest();
    request.open("GET", url);
    request.onreadystatechange = function() {
        if (this.readyState == 4)
            callback(this.responseText);
    };
    request.send(null);
}

var n = 2;
getText('/file.txt', function(text) {
    var numbers = text.split(' '); // text.split(' ').map(Number)
    var contains = numbers.indexOf(String(n)) > -1;
    alert(contains ? 'Ага' : 'Неа'); 
});