Detecting a string in JavaScript

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;

};

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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, );

}