Here is an example:

var clientContext = SP.ClientContext.get_current();
var list = clientContext.get_web().get_lists().getByTitle("Title");
var item = list.getItemById(id);
var fieldValuesAsText = item.get_fieldValuesAsText();

clientContext.load(fieldValuesAsText);
clientContext.executeQueryAsync(function (sender, args) {
    var fieldValues = fieldValuesAsText.get_fieldValues(); // Text values of all fields
    var fieldValue = fieldValuesAsText.get_item('Title'); // Text value of one field
}, function (sender, args) {
    alert(args.get_message());
});

Here is an example:

var clientContext = SP.ClientContext.get_current();
var list = clientContext.get_web().get_lists().getByTitle("Title");
var item = list.getItemById(id);
var fieldValuesAsText = item.get_fieldValuesAsText();

clientContext.load(fieldValuesAsText);
clientContext.executeQueryAsync(function (sender, args) {
    var fieldValues = fieldValuesAsText.get_fieldValues(); // Text values of all fields
    var fieldValue = fieldValuesAsText.get_item('Title'); // Text value of one field
}, function (sender, args) {
    alert(args.get_message());
});

Update, get field values as text of all items in one executeQueryAsync call:

var clientContext = SP.ClientContext.get_current();
var list = clientContext.get_web().get_lists().getByTitle("Title");
var items = list.getItems(new SP.CamlQuery());

clientContext.load(items, 'Include(FieldValuesAsText)');
clientContext.executeQueryAsync(function (sender, args) {
    var enumerator = items.getEnumerator();

    while (enumerator.moveNext()) {
        var item = enumerator.get_current();
        var fieldValuesAsText = item.get_fieldValuesAsText();
        var fieldValues = fieldValuesAsText.get_fieldValues();
        console.log(fieldValues);
    }
}, function (sender, args) {
    alert(args.get_message());
});