1var str1 = "Hello ";
2var str2 = "world!";
3var res = str1.concat(str2);
4console.log(res);

1
2var str1 = "Hello ";
3var str2 = "world!";
4var res = str1.concat(str2);
5// does not change the existing strings, but
6// returns a new string containing the text
7// of the joined strings.

1//This method adds two or more strings and returns a new single string.
2
3let str1 = new String( "This is string one" ); 
4let str2 = new String( "This is string two" ); 
5let str3 = str1.concat(str2.toString());
6console.log("str1 + str2 : "+str3)
7
8output:
9str1 + str2 : This is string oneThis is string two

1var aString="";
2aString.concat(value1, value2, ... value_n);