What is the usage of adding an empty string in a javascript statement

Type Casting. It converts the type to string

If variable current.condition_field is not of string type, by adding '' using + operator at the end/beginning of it converts it to string.

var field = current.condition_field + ''; 

So, field is always string.

Example

var bool = true; // Boolean
var str = bool + ''; // "true"

document.write('bool: ' + typeof bool + '<br />str: ' + typeof str);


var num = 10; // Numeric
var str = num + ""; // "10"

document.write('<br /><br />num: ' + typeof num + '<br />str: ' + typeof str);

Thanks to @KJPrice:

This is especially useful when you want to call a string method(Method defined on string prototype) on that variable.

(myVar + '').toLowerCase();