JavaScript is a programming language that compiles to bytecode. It can be treated as object code, but it is a very different and higher-level thing than the term "bytecode" generally denotes.

Calling JavaScript "flexible bytecode" is like saying that all Turing-complete programming languages are the same, or that all objects capable of driving a nail into a piece of wood are equivalent. It's true in a very facile way. JavaScript can be treated kind of like bytecode, and all Turing-complete languages can be treated as a simulation of a Turing machine, but the details that these rough descriptions miss are more numerous and more significant than the details that actually went into forming the generalization.