Technically speaking, HTML 5 introduced web-workers which allows javascript to have multiple threads (see 1). However, it's still useful to think about them as being single threaded since workers are sandboxed, meaning they don't have access to the global namespace. This avoids issues commonly found in concurrent programming. E.g., separate threads cannot read and write to the same variable since they don't have access to each other's namespaces.

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/Performan...