So, a code interpreter is a program that performs three main actions:

1. Analyzes the code line by line.
2. Processes that code.
3. Executes the original code.

When the interpreter runs code line by line, it analyzes and executes commands on the fly, without prior compilation. This offers one major advantage — instant feedback. That’s why browsers originally used JavaScript interpreters: the faster the code starts running, the better the user experience.

But there are also downsides. The interpreter has to reprocess the same code each time it appears. For example, inside a loop, it re-analyzes the same lines over and over again, leading to redundant computations. There’s no optimization — just mechanical execution of commands.

Another drawback: errors are only detected at the moment of execution. If there is a bug in the code, but the corresponding line hasn’t been reached yet, the interpreter won’t warn about it in advance. Compilers, unlike interpreters, can catch errors during the code-to-machine translation phase.

This approach makes interpreters convenient for quick debugging and interactive programming, but less efficient for running complex calculations.