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And none of those solve the issue.

You pass "foo" to Template. The Template will be instantiated before log ever sees it. You conveniently left out where the Foo string is computed from something that actually need computation.

Like both:

    new Template("doing X to " + thingBeingOperatedOn)

    new Template("doing " + expensiveDebugThing(thingBeingOperatedOn))

You just complicated everything to get the same class of error.

Heck even the existing good way of doing it, which is less complicated than your way, still isn't safe from it.

    logger("doing {}", expensiveDebugThing(thingBeingOperatedOn))

All your examples have the same issue, both with just string concatenation and more expensive calls. You can only get around an unknowing or lazy programmer if the compiler can be smart enough to entirely skip these (JIT or not - a JIT would need to see that these calls never amount to anything and decide to skip them after a while. Not deterministically useful of course).

Yeah, it's hard to prevent a sufficiently motivated dev from shooting itself in the foot; but these still help.

> You conveniently left out where the Foo string is computed from something that actually need computation.

I left it out because the comment I was replying to was pointing that some logs don't have params.

For the approach using a `Template` class, the expectation would be that the doc would call out why this class exists in the first place as to enable lazy computation. Doing string concatenation inside a template constructor should raise a few eyebrows when writing or reviewing code.

I wrote `logger.log(new Template("foo"))` in my previous comment for brevity as it's merely an internet comment and not a real framework. In real code I would not even use stringy logs but structured data attached to a unique code. But since this thread discusses performance of stringy logs, I would expect log templates to be defined as statics/constants that don't contain any runtime value. You could also integrate them with metadata such as log levels, schemas, translations, codes, etc.

Regarding args themselves, you're right that they can also be expensive to compute in the first place. You may then design the args to be passed by a callback which would allow to defer the param computation.

A possible example would be:

    const OPERATION_TIMEOUT = new Template("the operation $operationId timed-out after $duration seconds", {level: "error", code: "E_TIMEOUT"});
    // ...
    function handler(...) {
      // ..
      logger.emit(OPERATION_TIMEOUT, () => ({operationId: "foo", duration: someExpensiveOperationToRetrieveTheDuration()}))
    }

This is still not perfect as you may need to compute some data before the log "just in case" you need it for the log. For example you may want to record the current time, do the operation. If the operation times out, you use the time recorded before the op to compute for how long it ran. If you did not time out and don't log, then getting the current system time is "wasted".

All I'm saying is that `logger.log(str)` is not the only possible API; and that splitting the definition of the log from the actual "emit" is a good pattern.