The following engines use runtime interpreters, which do not compile into native machine code and generally run more slowly.
Continuum: A self-interpreter that supports recent iterations of the draft ECMAScript 6 specification. Uniquely, the engine is implemented in ECMAScript 3, which makes it possible to run ES6 in browsers as old as IE6.
Futhark: The ECMAScript engine of the Opera web browser versions 9.50 to 10.10.
YAJI: An ECMAScript engine based on the FESI implementation by Jean-Marc Lugrin in 1999, using the Java platform, currently being developed to support the latest standards (Spec 262, v5.1).
Duktape: a small footprint, easily embeddable Ecmascript E5/E5.1 engine.
The Kinoma Platform, an ECMAScript 5 runtime environment and framework