feat(handler): implement StreamHandler, ConsoleHandler, StderrHandler emit

StreamHandler.emit() writes formatted output to a Writable stream.
ConsoleHandler maps log levels to console.warn/error/log. StderrHandler
emits via console.error. handleError() logs diagnostics instead of
throwing. FileHandler throws NotImplementedError on construction.
Removed Node.js stream import and conditional require() block.

src/handler.ts

@@ -1,17 +1,9 @@
-import * as stream from 'stream';
-
-import { LogLevel, checkLevel, NOTSET } from './log-level';
+import { LogLevel, checkLevel, NOTSET, WARNING, ERROR } from './log-level';
 import { LogRecord } from './log-record';
 import { Formatter, DEFAULT_FORMATTER } from './formatter';
 import { Filterer } from './filter';
 import { NotImplementedError } from './helper/error';
-
-if (typeof window === 'undefined') {
-    const stream = require('stream');
-}
-else {
-    const stream = require('./helper/stream');
-}
+import { Writable, DEFAULT_STREAM, StderrWritable } from './helper/stream';
 
 //---------------------------------------------------------------------------
 // Handler classes and functions
@@ -115,14 +107,13 @@ export class Handler extends Filterer {
      * I/O thread lock.
      */
      handle(record: LogRecord) {
-        var rv = this.filter(record);
+        const rv = this.filter(record);
+        if (!rv) { return }
+        let filtered = record;
         if ((rv as any) instanceof LogRecord) {
-            record = rv as unknown as LogRecord
-        }
-        if (rv) {
-            //locking here
-            this.emit(record)
+            filtered = rv as unknown as LogRecord
         }
+        this.emit(filtered)
     }
 
     /**
@@ -144,23 +135,73 @@ export class Handler extends Filterer {
      * Handle errors which occur during an emit() call.
      *
      * This method should be called from handlers when an exception is
-     * encountered during an emit() call. If raiseExceptions is false,
-     * exceptions get silently ignored. This is what is mostly wanted
-     * for a logging system - most users will not care about errors in
-     * the logging system, they are more interested in application errors.
-     * You could, however, replace this with a custom handler if you wish.
-     * The record which was being processed is passed in to this method.
+     * encountered during an emit() call.
      */
     handleError(record: LogRecord) {
-        throw new NotImplementedError(
-            'still need to find portable way for stacktracing...'
-        )
+        try {
+            console.error('--- Logging error ---');
+            console.error('Error in handler for record:', record.scope, record.msg);
+        }
+        catch (_) {
+            // silently ignore errors in error handling
+        }
     }
 
     get formatter(): Formatter|null { return this._formatter }
     set formatter(fmt: Formatter) { this._formatter = fmt }
 }
 
+/**
+ * A handler class which writes logging records, appropriately formatted,
+ * to a stream. Note that this class does not close the stream, as
+ * the default stream may be shared.
+ */
+export class StreamHandler extends Handler {
+    protected stream: Writable;
+
+    constructor(stream?: Writable) {
+        super();
+        this.stream = stream ?? DEFAULT_STREAM;
+    }
+
+    emit(record: LogRecord) {
+        try {
+            const msg = this.format(record);
+            this.stream.write(msg);
+        }
+        catch (e) {
+            this.handleError(record);
+        }
+    }
+}
+
+/**
+ * A handler class which writes logging records to the browser console,
+ * mapping log levels to the appropriate console method.
+ *
+ * This is the primary handler for browser environments.
+ */
+export class ConsoleHandler extends Handler {
+    emit(record: LogRecord) {
+        try {
+            const msg = this.format(record);
+
+            if (record.levelno >= ERROR) {
+                console.error(msg);
+            }
+            else if (record.levelno >= WARNING) {
+                console.warn(msg);
+            }
+            else {
+                console.log(msg);
+            }
+        }
+        catch (e) {
+            this.handleError(record);
+        }
+    }
+}
+
 export interface FileHandlerOptions {
     filename: string
     filemode?: string
@@ -168,20 +209,13 @@ export interface FileHandlerOptions {
     errors?: string
 }
 
-/**
- * A handler class which writes logging records, appropriately formatted,
-   to a stream. Note that this class does not close the stream, as
-   sys.stdout or sys.stderr may be used.
- */
-export class StreamHandler extends Handler {
-    constructor(stream?: stream.Writable) {
-        super();
-    }
-}
-
 export class FileHandler extends StreamHandler {
     constructor(options: FileHandlerOptions) {
         super();
+        throw new NotImplementedError(
+            'FileHandler is not available in browser environments. ' +
+            'Use ConsoleHandler or a storage-backed handler instead.'
+        );
     }
 }
 
@@ -191,8 +225,15 @@ export class FileHandler extends StreamHandler {
  * sys.stderr at handler construction time.
  */
 export class StderrHandler extends Handler {
-    /**
-     * Initialize the handler.
-     */
     constructor(level: LogLevel) { super(level) }
+
+    emit(record: LogRecord) {
+        try {
+            const msg = this.format(record);
+            console.error(msg);
+        }
+        catch (e) {
+            this.handleError(record);
+        }
+    }
 }