如何访问文件处理程序以使用 Typescript 在 deno 日志模块中运行刷新方法

Question

I need to access the fileHandler object of my logger so I can flush the buffer to the file.

This is my program:

import * as log from "https://deno.land/std@0.75.0/log/mod.ts"
import { Application } from "https://deno.land/x/oak@v6.3.1/mod.ts";

const app   = new Application()
const port  = 7001

await log.setup({
    handlers:{
        file: new log.handlers.FileHandler("DEBUG",{
            filename: "logger.log",
            formatter: lr => {
                return `${lr.datetime.toISOString()} [${lr.levelName}] ${lr.msg}`
            }
        })
    },
    loggers: {
        default: {
            level: "DEBUG",
            handlers: ["file"]
        }
    }
})
const logger = log.getLogger()
logger.debug("hi there")

app.use((ctx) => {
    ctx.response.body = 'Hi there'
})

console.log(`listening on port ${port}`)
app.listen({ port })

My problem is that the log message is never being written to file. If I remove the last line ( app.listen() ) it Does write to the file because the process ends. But if I leave it listening process never ends so the log buffer is never flushed.

If I interrupt the process with Ctrl-C it doesn't write it either

Documentation ( https://deno.land/std@0.75.0/log/README.md ) says I can force log flush using the flush method from FileHandler. But I don't know how to access the fileHandler object.
So I've tried this:

const logger = log.getLogger()
logger.debug("hi there")
logger.handlers[0].flush()

And it works! but only as javascript, NOT as typescript
As typescript I get this error:

error: TS2339 [ERROR]: Property 'flush' does not exist on type 'BaseHandler'.
logger.handlers[0].flush()

Answer 1

Well, I found a solution.
I just have to import the FileHandler class and cast my handler down from BaseHandler to FileHandler. So I added this line among the imports:

import { FileHandler } from "https://deno.land/std@0.75.0/log/handlers.ts"

And then after creating the logger:

logger.debug("hi there")
const fileHandler = <FileHandler> logger.handlers[0]
fileHandler.flush()

Looks a little weird, I still guess there must be less quirky / more semantic solution for this. But it works ok.

Answer 2

Let us just recap with the help of Santi's answer.

In my experience logs in file work fine in an ending program. I mean a program which dies by itself or with Deno.exit(0). Problem occurs in a never ending loop. In this case logs don't append in their files. Below is how to overcome this situation :

// dev.js : "I want my logs" example
import {serve} from "https://deno.land/std@0.113.0/http/server_legacy.ts";
import * as log from "https://deno.land/std@0.113.0/log/mod.ts";

// very simple setup, adapted from the official standard lib https://deno.land/std@0.113.0/log
await log.setup({
  handlers: {
    file: new log.handlers.FileHandler("WARNING", {
      filename: "./log.txt",
      formatter: "{levelName} {msg}",
    }),
  },
  loggers: {
    default: {
      level: "DEBUG",
      handlers: ["file"],
    },
  },
});

// here we go
let logger;
logger = log.getLogger();
logger.warning('started');  

const fileHandler = logger.handlers[0];
await fileHandler.flush(); // <---- the trick, need to flush ! Thanks Santi

// loop on requests
const srv = serve(`:4321`);
for await (const request of srv) {
  request.respond({body: 'bonjour', status: 200});

  logger.warning('hit !');
  fileHandler.flush();  // <---- flush again
}

Run with

$ deno run -A dev.js

And check the file log.txt with the following trigger

$ curl localhost:4321

This is a very low tech, problably adding important delay to the process. The next level will be to fire a time event to flush every minute or so.