JavaScript Detecting a string


Example

To detect whether a parameter is a primitive string, use typeof:

var aString = "my string";
var anInt = 5;
var anObj = {};
typeof aString === "string";   // true
typeof anInt === "string";     // false
typeof anObj === "string";     // false

If you ever have a String object, via new String("somestr"), then the above will not work. In this instance, we can use instanceof:

var aStringObj = new String("my string");
aStringObj instanceof String;    // true

To cover both instances, we can write a simple helper function:

var isString = function(value) {
    return typeof value === "string" || value instanceof String;
};

var aString = "Primitive String";
var aStringObj = new String("String Object");
isString(aString); // true
isString(aStringObj); // true
isString({}); // false
isString(5); // false

Or we can make use of toString function of Object. This can be useful if we have to check for other types as well say in a switch statement, as this method supports other datatypes as well just like typeof.

var pString = "Primitive String";
var oString = new String("Object Form of String");
Object.prototype.toString.call(pString);//"[object String]"
Object.prototype.toString.call(oString);//"[object String]"    

A more robust solution is to not detect a string at all, rather only check for what functionality is required. For example:

var aString = "Primitive String";
// Generic check for a substring method
if(aString.substring) {

}
// Explicit check for the String substring prototype method
if(aString.substring === String.prototype.substring) {
    aString.substring(0, );
}