What’s the point of new String(“x”) in JavaScript?

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There’s very little practical use for String objects as created by new String("foo"). The only advantage a String object has over a primitive string value is that as an object it can store properties:

var str = "foo";
str.prop = "bar";
alert(str.prop); // undefined

var str = new String("foo");
str.prop = "bar";
alert(str.prop); // "bar"

If you’re unsure of what values can be passed to your code then I would suggest you have larger problems in your project. No native JavaScript object, major library or DOM method that returns a string will return a String object rather than a string value. However, if you want to be absolutely sure you have a string value rather than a String object, you can convert it as follows:

var str = new String("foo");
str = "" + str;

If the value you’re checking could be any object, your options are as follows:

  1. Don’t worry about String objects and just use typeof. This would be my recommendation.

    typeof str == "string".

  2. Use instanceof as well as typeof. This usually works but has the disadvantage of returning a false negative for a String object created in another window.

    typeof str == "string" || str instanceof String

  3. Use duck typing. Check for the existence of one or more String-specific methods, such as substring() or toLowerCase(). This is clearly imprecise, since it will return a false positive for an object that happens to have a method with the name you’re checking, but it will be good enough in most cases.

    typeof str == "string" || typeof str.substring == "function"

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