JavaScript string initialization

Question

In JavaScript, from my understanding, the below are all same:

var carter2 = new String();
var carter2 = '';
var carter2 = "";

Which one is the most preferred?

Answer 1

Don't use

var str = new String();

Because

var str = new String("dog");
var str2 = new String("dog"); 
str == str2; // false

But

var str = "dog";
var str2 = "dog"; 
str == str2; // true

However, because of type coercion, the following works (Thanks to Rocket for pointing it out)

var str = new String("dog");
var str2 = "dog"; 
str == str2; // true

Single and double quotes don't matter, except for what quotes need to be escaped. Many others have noted that single quotes are better when creating HTML strings, since XHTML expects attributes to have double quotes, and you won't need to escape them.

Answer 2

When doing new String(), you are not being returned a string primitive, but a String object.

For all intents and purposes it acts like a string primitive, but there are cases where it won't. For example:

var a = new String('body');
jQuery(a); // This will *not* function as expected
           // This will make a jQuery object containing a "String" object
           // It will NOT treat it as a selector string

Also when comparing things, there may be problem. When you compare objects in JavaScript, it's only true if they are the same exact object, not just the same value. Example:

var a = new String('test');
var b = new String('test');
var c = a;
var d = 'test';

a === b; // false, a and b are different objects (a == b is also false)
c === a; // true, a is the same object as c
c === b; // false, c (which is a) is a different object than b
d === a; // false, d is a primitive and a is an object, they are not equal
'test' === d; // true, they are both string primitives

d == a; // true, this uses "==" so only value is compared, not type

You can use .valueOf() to convert a String object to a string primitive.

new String('test').valueOf() === 'test'; // true

So, I highly suggest using var a = '' or var a = "". As for single quotes vs. double quotes, there is no difference. Consider this example:

var a = "My name is Joe O'Ryan";
var b = 'My name is Joe O\'Ryan'; // need to escape the '
var c = 'Joe said: "Hi"';
var d = "Joe said \"HI\""; // need to escape the "

So, it's up to you whether to use ' or ", but I suggest those over new String().