MDN has the following to say about string.concat():
It is strongly recommended to use the string concatenation operators
(+, +=) instead of this method for perfomance reasons
Also see the link by @Bergi.
In JS, “+” concatenation works by creating a new String object.
For example, with…
var s = “Hello”;
…we have one object s.
Next:
s = s + ” World”;
Now, s is a new object.
2nd method:
String.prototype.concat