Out of memory when concatinating strings

//strings.js

var A="1234567890";
var B=A;
console.log(B);console.log(B.length);
B=B+B;
console.log(B);console.log(B.length);
//out of memory if continued
//B=B+B;
//console.log(B);console.log(B.length);
//B=B+B;
//console.log(B);console.log(B.length);

B=A;
console.log(B);console.log(B.length);
B+=B;
console.log(B);console.log(B.length);
B+=B;
console.log(B);console.log(B.length);

Produces:

>echo(0);
1234567890
10
12345678901234567890
20
1234567890
10
12345678901234567890
20
ERROR: Out of Memory!
WARNING: Out of memory while appending to array
Execution Interrupted
=undefined
>