probe.onLine

Fire when execution reaches a line, without stopping it.

This is the target that reaches code no name can address: a local inside a module, a helper a component never exported, a branch you want to know is taken. path is the script's chunk name, VFS path, or require identity, and line snaps to the next executable line.

probe.onLine("Ticker", 9, {
    notice = "score got big",
    when   = "doubled >= 6",
    run    = "return locals.doubled",
})

The world is not stopped. The line runs on, and the body sees the frame it was in: locals and upvalues are that frame's, read at the moment of the hit.

Where to put a comparison. when is evaluated inside that frame, so it reads the frame's own variables by their own names and with their own types — doubled >= 6. locals in the body is that frame rendered as text, which is what you want for reporting and not what you want for arithmetic. Compare in when, report from locals. A hit whose condition does not hold never reaches the body at all.

Set one on a script that has not loaded yet and it waits, then installs itself when the script loads.

The cost worth knowing. An unarmed line costs the virtual machine nothing, but a breakpoint drops the function containing it out of native code until that script reloads, and cancelling the probe does not bring native code back. On a function that runs every frame that is a real cost, so cancel when you are done rather than leaving it armed.