Filters allows you to filter out log records from your streams. A log record can be filtered out from one stream but not another. Filters are processed after monitors, but before transformers or stream handling. Upon handling a log record, the logger will run filters once for each registered stream.

Some use cases for filters include:

• Preventing spam from filling up your logs
• Removing log records which contain illegal characters
• Blocking malicious log records

There are two ways to construct a filter.

This is a good choice for short and simple filters. Filter functions must match the following type:

export type FilterFn = (stream: Stream, logRecord: LogRecord) => boolean;

The function takes in a stream and logRecord and returns true if the logRecord should be filtered out. Example:

const filter: FilterFn = (stream: Stream, logRecord: LogRecord) =>
  (logRecord.msg as string).includes("bad stuff");

The Filter interface requires you to implement the shouldFilterOut function, which is of type FilterFn as above. This gives you the power of a class for more complex filtering, or perhaps you want to redirect filtered out logs to another logger and stream.

class MyFilter implements Filter {
  shouldFilerOut(stream: Stream, logRecord: LogRecord): boolean {
    return (logRecord.msg as string).includes("bad stuff");
  }
}

Filters are registered directly with the logger as follows:

const myFilter = new MyFilter();
const logger = new Logger().addFilter(myFilter);

Two filters are available in Optic.

This filter takes in a regular expression. If it matches, then the log record is filtered out. The log record msg and metadata fields are first converted to a string if necessary before testing the regular expression.

// Filters out log records containing `%` or `&` in the message or metadata
import { RegExpFilter } from "https://deno.land/x/optic/filters/regExpFilter.ts";

const regExpFilter = new RegExpFilter(/[%&]+/);
const logger = new Logger().addFilter(regExpFilter);
logger.error("Oh no!"); // not filtered
logger.error("Oh no!", "& another thing"); // filtered out

This filter takes in a string. If this string is found to be a substring of either the log record msg or metadata fields (converting them to string first if required), then this log record is filtered out. Example:

import { SubStringFilter } from "https://deno.land/x/optic/filters/subStringFilter.ts";
import { Logger } from "https://deno.land/x/optic/mod.ts";

const subStringFilter = new SubStringFilter("user1234");
const logger = new Logger().addFilter(subStringFilter);
logger.info({ user: "joe1944", action: "login" }); // not filtered
logger.info({ user: "user1234", action: "login" }); // filtered out