To string conversions

Note: the comment on the right side is the return value of the left side statement.

String('foo')                 // foo
JSON.stringify('foo')         // "foo"
'foo'.toString()              // foo

String({})                    // [object Object]
JSON.stringify({})            // {}
{}.toString()                 // [object Object]

String(null)                  // null
JSON.stringify(null)          // null
null.toString()               // Cannot read property 'toString' of null

String(undefined)             // undefined
JSON.stringify(undefined)     // undefined
undefined.toString()          // Cannot read property 'toString' of undefined

null and undefined are not object. Ok null is but it should not be. Nevertheless both lack a toString method, hence the: Cannot read property 'toString'.

const obj = {
  baz: 'baz',
  toString: function () { return 'foo'; },
  valueOf: function () { return 'bar'; }
};

String(obj)                  // foo
JSON.stringify(obj)          // {"baz:"baz}
obj.toString()               // foo

obj + 'qux'                  // barqux
const arr = [obj, 'qux'];
arr[0] + arr[1]              // fooqux

• We can see that the String function calls the toString method behind the scene.
• When using the + operator the concatenation uses the valueOf method.
• On the other hand [obj, 'qux'] is calling the toString method.

obj.toString = null;

String(obj)                  // bar
JSON.stringify(obj)          // {"baz:"baz,"toString":null}
obj.toString()               // obj.toString is not a function

The toString method is not available so the String function falls back to the valueOf method.

obj.valueOf = null;

String(obj)                  // Cannot convert object to primitive value
JSON.stringify(obj)          // {"baz:"baz,"toString":null,"valueOf":null}
obj.toString()               // obj.toString is not a function

None of the needed methods are here, it throws.

const obj = {
  baz: 'baz',
  toString: function () { return 'foo'; },
  valueOf: function () { return 'bar'; }
};

obj.valueOf = null;

obj + 'qux'                  // fooqux
const arr = [obj, 'qux'];
arr[0] + arr[1]              // fooqux

Funny enough, or not, the concatenation with the + operator uses toString as a fallback if valueOf is not available. It's the exact opposite behavior as with the string conversion by the String function.

Source: http://www.adequatelygood.com/Object-to-Primitive-Conversions-in-JavaScript.html