Is it possible to capture console log output?
I see that console.log calls out to core.op_print. Is it possible to patch an object so that I can capture log output? Similar to how one might patch process.stdout.write with node?
5 Replies
Yes, it is possible to capture console log output in Deno by customizing the
console object. The documentation provides an example of how to create a custom JavaScript runtime that includes a simplified console API. In this example, the console.log and console.error functions are implemented to call Deno.core.print, which is a function provided by the deno_core Rust crate.
Here's a snippet from the documentation that shows how the console object is defined:
In this code, console.log and console.error are overridden to format the arguments and then call core.print. If you want to capture the log output, you could modify this implementation to store the messages in a variable or handle them in a different way instead of printing them directly.
For example, you could create a custom logger like this:
``javascript
((globalThis) => {
const core = Deno.core;
let capturedLogs = [];
function argsToMessage(...args) {
return args.map((arg) => JSON.stringify(arg)).join(" ");
}
function captureLog(message) {
capturedLogs.push(message);
// You can also perform additional actions here, like sending the logs to a server
}
globalThis.console = {
log: (...args) => {
const message = [out]: ${argsToMessage(...args)}\n;
captureLog(message);
},
error: (...args) => {
const message = [err]:${argsToMessage(...args)}\n
;
captureLog(message);
},
getCapturedLogs: () => capturedLogs,
};
})(globalThis);
```
With this custom logger, you can call console.getCapturedLogs() to retrieve the captured log messages. This is similar to patching process.stdout.write in Node.js, but adapted for the Deno environment and the deno_core` crate. Remember that this is a simplified example, and in a real-world scenario, you might need to handle concurrency and other considerations depending on your use case. 🦕Deno.core doesn't exist
You are correct; the
Deno.core namespace is not part of the stable Deno API and should not be used for capturing console log output. My previous message included an incorrect reference to Deno.core, which is not applicable.
To capture console log output in Deno, you would typically have to override the console.log function with your own implementation that captures the output. However, the documentation does not provide a direct example of how to capture console log output.
As an alternative, you could consider writing a custom logger function that you use throughout your application instead of console.log. This custom logger could then store the logs in a buffer or perform any other action you need.
Here's a simple example of a custom logger:
In this example, customLogger is a function that captures log messages in an array and also prints them to the console. You would use customLogger instead of console.log throughout your application to capture the logs.
Please note that this is a general advice and not a direct example from the documentation. If you need to capture logs from third-party libraries that use console.log directly, you would need to override console.log globally, which is generally not recommended due to potential side effects on logging behavior across your application and libraries. 🦕🦕💚 I'm glad I was able to help!