javascript - Why is there "undefined" text in the beginning of my string?

Question

I have a function that concatenates together the results from an AJAX request.

For some reason, my final string starts with "undefined".

Here is a simplified example that reproduces the problem:

    // In practice, fetched via AJAX from a server
    var vendors = [{ id_vendor: 'V0001' }, { id_vendor: 'V0002' }];

    var row_vendor;

    vendors.forEach(function (value) {
      row_vendor += value.id_vendor;
    });

    alert(row_vendor); // undefinedV0001V0002

Answer 1

You're not initializing your variable, so its value is undefined. Concatenating a string coerces it to the string "undefined" before the concatenation.

Consider:

var x
alert(x + "test") // undefinedtest

Instead, initialize your variable to an empty string before performing concatenation:

var x = ""
alert(x + "test") // test

Note that functionally it is much cleaner to first extract the property you're interested in and then simply join them together:

$.map(vendor, function (v) { return v.vendor_id }).join('')