# Blockly > Demos > Step Execution with JS Interpreter

This is a demo of executing code step-by-step with a sandboxed JavaScript interpreter.

The generator's `Blockly.JavaScript.STATEMENT_PREFIX` is assigned `'highlightBlock(%1);\n'`, where `%1` is the block id. The call to `highlightBlock()` will highlight the identified block and set the variable `highlightPause` to `true`.

"Parse JavaScript" will generate the code and load it into the interpreter. Then, each press of the "Step JavaScript" button will run the interpreter one step until the `highlightPause` is true. That is, until `highlightBlock()` has highlighted the block that will be executed on the next step.

→ More info on running code with JS Interpreter

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