What's the difference between String(value) vs value.toString()

They are not completely the same, and actually, the String constructor called as a function (your first example), will at the end, call the toString method of the object passed, for example:

var o = { toString: function () { return "foo"; } };
String(o); // "foo"

On the other hand, if an identifier refers to null or undefined, you can't use the toString method, it will give you a TypeError exception:

var value = null;
String(null);     // "null"
value.toString(); // TypeError

The String constructor called as a function would be roughly equivalent to:

value + '';

The type conversion rules from Object-to-Primitive are detailed described on the specification, the [[DefaultValue]] internal operation.

Briefly summarized, when converting from Object-to-String, the following steps are taken:

1. If available, execute the toString method.
   • If the result is a primitive, return result, else go to Step 2.
2. If available, execute the valueOf method.
   • If the result is a primitive, return result, else go to Step 3.
3. Throw TypeError.

Given the above rules, we can make an example of the semantics involved:

var o = {
  toString: function () { return "foo"; },
  valueOf:  function () { return "bar"; }
};

String(o); // "foo"

// Make the toString method unavailable:
o.toString = null;

String(o); // "bar"

// Also make the valueOf method unavailable:
o.valueOf = null;

try { 
  String(o); 
} catch (e) {
  alert(e); // TypeError
}

If you want to know more about this mechanism I would recommend looking at the ToPrimitive and the ToString internal operations.

I also recommend reading this article:

• Object-to-Primitive Conversions in JavaScript

value.toString() will cause an error if value is null or undefined. String(value) should not.

For example:

var value = null;
alert(value.toString());

will fail because value == null.

var value = null;
alert(String(value));

should display a message reading "null" (or similar), but it will not crash.