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141 lines
3.9 KiB
Markdown
141 lines
3.9 KiB
Markdown
# Log
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## Usage
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```ts
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import * as log from "https://deno.land/std/log/mod.ts";
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// simple default logger, you can customize it
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// by overriding logger and handler named "default"
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log.debug("Hello world");
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log.info("Hello world");
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log.warning("Hello world");
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log.error("Hello world");
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log.critical("500 Internal server error");
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// custom configuration
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await log.setup({
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handlers: {
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console: new log.handlers.ConsoleHandler("DEBUG"),
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file: new log.handlers.FileHandler("WARNING", {
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filename: "./log.txt",
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// you can change format of output message
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formatter: "{levelName} {msg}"
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})
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},
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loggers: {
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// configure default logger available via short-hand methods above
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default: {
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level: "DEBUG",
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handlers: ["console", "file"]
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},
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tasks: {
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level: "ERROR",
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handlers: ["console"]
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}
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}
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});
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let logger;
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// get default logger
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logger = log.getLogger();
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logger.debug("fizz"); // logs to `console`, because `file` handler requires "WARNING" level
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logger.warning("buzz"); // logs to both `console` and `file` handlers
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// get custom logger
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logger = log.getLogger("tasks");
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logger.debug("fizz"); // won't get output becase this logger has "ERROR" level
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logger.error("buzz"); // log to `console`
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// if you try to use a logger that hasn't been configured
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// you're good to go, it gets created automatically with level set to 0
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// so no message is logged
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unknownLogger = log.getLogger("mystery");
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unknownLogger.info("foobar"); // no-op
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```
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## Advanced usage
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### Loggers
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Loggers are objects that you interact with. When you use logger method it constructs a `LogRecord` and passes it down to its handlers for output. To create custom loggers speficify them in `loggers` when calling `log.setup`.
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#### `LogRecord`
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`LogRecord` is an object that encapsulates provided message and arguments as well some meta data that can be later used when formatting a message.
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```ts
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interface LogRecord {
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msg: string;
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args: any[];
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datetime: Date;
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level: number;
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levelName: string;
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}
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```
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### Handlers
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Handlers are responsible for actual output of log messages. When handler is called by logger it firstly checks that `LogRecord`'s level is not lower than level of the handler. If level check passes, handlers formats log record into string and outputs it to target.
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`log` module comes with two built-in handlers:
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- `ConsoleHandler` - (default)
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- `FileHandler`
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#### Custom message format
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If you want to override default format of message you can define `formatter` option for handler. It can be either simple string-based format that uses `LogRecord` fields or more complicated function-based one that takes `LogRecord` as argument and outputs string.
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Eg.
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```ts
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await log.setup({
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handlers: {
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stringFmt: new log.handlers.ConsoleHandler("DEBUG", {
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formatter: "[{levelName}] {msg}"
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}),
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functionFmt: new log.handlers.ConsoleHandler("DEBUG", {
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formatter: logRecord => {
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let msg = `${logRecord.level} ${logRecord.msg}`;
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logRecord.args.forEach((arg, index) => {
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msg += `, arg${index}: ${arg}`;
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});
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return msg;
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}
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}),
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},
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loggers: {
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default: {
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level: "DEBUG",
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handlers: ["stringFmt", "functionFmt"],
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},
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}
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})
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// calling
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log.debug("Hello, world!", 1, "two", [3, 4, 5]);
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// results in:
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[DEBUG] Hello, world! // output from "stringFmt" handler
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10 Hello, world!, arg0: 1, arg1: two, arg3: [3, 4, 5] // output from "functionFmt" formatter
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```
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#### Custom handlers
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Custom handlers can be implemented by subclassing `BaseHandler` or `WriterHandler`.
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`BaseHandler` is bare-bones handler that has no output logic at all,
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`WriterHandler` is an abstract class that supports any target with `Writer` interface.
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During setup async hooks `setup` and `destroy` are called, you can use them to open and close file/HTTP connection or any other action you might need.
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For examples check source code of `FileHandler` and `TestHandler`.
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