Question

I know you can concatenate a string like this in Javascript:

str1 + str2;

Or like this:

[str1, str2].join('');

Why is there a function to do this, too?

str1.concat(str2)

Is it any faster? Just seems like more typing to me. My guess is that this was used before str1 + str2 was optimized. Would that be correct?

Answer 1

The String.prototype.concat method is very generic (ECMA Standard 5.1 Edition / June 2011, pp.143-144):

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let args be an internal list that is a copy of the argument list passed to this function.
4. Let R be S.

5. Repeat, while args is not empty

   a. Remove the first element from args and let next be the value of that element.

   b. Let R be the String value consisting of the characters in the previous value of R followed by the
   characters of ToString(next).

6. Return R.

Because it’s so generic it can be used on almost every object that has a ToString() method. See also the following note:

NOTE:
The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

So you can use String.prototype.concat.call(myObject,....);. Note that there’s also a Array.prototype.concat method, which is also very generic (p. 125).