inspector: add --cond to node inspect probe mode#64328
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On a hot path, the probe can record every hit and require
filtering afterwards. This patch adds a per-probe `--cond <expr>`
option that allows limiting the hit to only when the expression
is truthy at the probe location. V8 evaluates it as the breakpoint's
native condition, so the target is not paused when it does not hold,
and a condition that throws is treated as false. Since in CDP,
a location can only carry one breakpoint per URL pattern,
probes sharing a location must share one condition (or none).
Conflicting conditions are rejected.
Example:
```js
// app.js
let total = 0;
for (let i = 0; i < 10; i++) {
total += i; // line 4
}
```
```
$ out/Release/node inspect --probe app.js:4 --expr 'total' \
--cond 'i % 3 === 0' app.js
```
```
Hit 1 at file:///path/to/app.js:3:3
total = 0
Hit 2 at file:///path/to/app.js:3:3
total = 3
Hit 3 at file:///path/to/app.js:3:3
total = 15
Hit 4 at file:///path/to/app.js:3:3
total = 36
Completed
```
Signed-off-by: Joyee Cheung <joyeec9h3@gmail.com>
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jasnell
reviewed
Jul 7, 2026
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| if (conditionByLocation.has(key)) { | ||
| if (conditionByLocation.get(key) !== condition) { |
Member
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I'm assuming this is structured into separate has/get checks because undefined is a legitimate value and differentiating between no-value and undefined is necessary? If so, a comment here would be helpful.
jasnell
reviewed
Jul 7, 2026
| @@ -321,6 +331,8 @@ $ node inspect --probe app.js:10 --expr "user" | |||
| --json --preview -- --no-warnings app.js --arg-for-app=foo | |||
| ``` | |||
|
|
|||
| <!-- TODO(joyeecheung): add more examples for different options --> | |||
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An example that shows --cond being used would be good :-)
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On a hot path, the probe can record every hit and require filtering afterwards. This patch adds a per-probe
--cond <expr>option that allows limiting the hit to only when the expression is truthy at the probe location. V8 evaluates it as the breakpoint's native condition, so the target is not paused when it does not hold, and a condition that throws is treated as false. Since in CDP, a location can only carry one breakpoint per URL pattern, probes sharing a location must share one condition (or none). Conflicting conditions are rejected.Example:
Refs: #63646