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

1const str1 = 'Hello';
2const str2 = 'World';
3
4console.log(str1.concat(' ', str2));
5// expected output: "Hello World"
6
7console.log(str2.concat(', ', str1));
8// expected output: "World, Hello"

1const str1 = 'Hello';
2const str2 = 'World';
3
4console.log(str1 + str2);
5>> HelloWorld
6
7console.log(str1 + ' ' + str2);
8>> Hello World

1var one = 'Hello';
2var two = 'World';
3
4console.log(one + two);
5It will print: HelloWorld
6
7console.log(one + ' ' + two);
8It will print: Hello World