Add browser task scheduler.

See #17 (comment)

Author: RubenVerborgh

.change-extension

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+# The first parameter is the desired extension
+extension=$1
+shift
+
+# The other parameters are the files to be renamed with the extension
+for file in "$@"; do
+  mv "$file" "${file%.*}.$extension"
+done

.eslintrc

@@ -106,6 +106,7 @@
     no-undef-init: error,
     no-undefined: off,
     no-unused-vars: [ error, { args: none } ],
+    "@typescript-eslint/no-unused-vars": [ error, { args: none } ],
     no-use-before-define: [ error, { functions: false, classes: false } ],
 
     // Node.js
@@ -229,7 +230,7 @@
     no-confusing-arrow: off,
     no-const-assign: error,
     no-dupe-class-members: error,
-    no-duplicate-imports: error,
+    no-duplicate-imports: off,
     no-new-symbol: error,
     no-restricted-imports: error,
     no-this-before-super: error,

.gitignore

@@ -3,8 +3,9 @@ coverage
 docs
 node_modules
 package
-*.log
-*.tgz
-*.d.ts
+*.bak
 *.cjs
+*.d.ts
+*.log
 *.mjs
+*.tgz

asynciterator.ts

@@ -4,12 +4,13 @@
  */
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
+import createTaskScheduler from './taskscheduler';
+import type { Task, TaskScheduler } from './taskscheduler';
 
-let taskScheduler: TaskScheduler = queueMicrotask;
+let taskScheduler: TaskScheduler = createTaskScheduler();
 
 /** Schedules the given ask for asynchronous execution. */
-export function scheduleTask(task: () => void): void {
+export function scheduleTask(task: Task): void {
   taskScheduler(task);
 }
 
@@ -1875,5 +1876,3 @@ type SourceExpression<T> =
 
 type InternalSource<T> =
   AsyncIterator<T> & { _destination: AsyncIterator<any> };
-
-type TaskScheduler = (task: () => void) => void;

package-lock.json

@@ -13,15 +13,17 @@
   "module": "./asynciterator.mjs",
   "sideEffects": false,
   "files": [
-    "asynciterator.mjs",
-    "asynciterator.cjs",
-    "asynciterator.d.ts"
+    "*.mjs",
+    "*.cjs",
+    "*.d.ts"
   ],
   "scripts": {
-    "build": "npm run build:module && npm run build:commonjs && npm run build:types",
-    "build:module": "tsc --module es2015 && mv asynciterator.js asynciterator.mjs",
-    "build:commonjs": "tsc --module commonjs && mv asynciterator.js asynciterator.cjs",
-    "build:types": "tsc -d && rm asynciterator.js",
+    "build": "npm run build:clean && npm run build:module && npm run build:commonjs && npm run build:types",
+    "build:clean": "rm -f *.js *.mjs *.cjs *.d.ts",
+    "build:module": "tsc --module es2015 && ./.change-extension mjs *.js && npm run build:fix-import",
+    "build:commonjs": "tsc --module commonjs && ./.change-extension cjs *.js",
+    "build:types": "tsc -d && rm *.js",
+    "build:fix-import": "sed -i'.bak' -e 's/taskscheduler/taskscheduler.mjs/' asynciterator.mjs",
     "prepare": "npm run build",
     "test": "npm run build:module && npm run test:microtask",
     "test:microtask": "npm run mocha",
@@ -33,9 +35,6 @@
   },
   "license": "MIT",
   "repository": "RubenVerborgh/AsyncIterator",
-  "dependencies": {
-    "queue-microtask": "^1.1.2"
-  },
   "devDependencies": {
     "@babel/cli": "^7.10.1",
     "@babel/core": "^7.10.2",

package.json

@@ -0,0 +1,24 @@
+const resolved = Promise.resolve(undefined);
+
+// Returns a function that asynchronously schedules a task
+export default function createTaskScheduler() : TaskScheduler {
+  // Use or create a microtask scheduler
+  const scheduleMicrotask = typeof queueMicrotask === 'function' ?
+    queueMicrotask : (task: Task) => resolved.then(task);
+
+  // If not in the browser, always use the microtask scheduler
+  if (typeof window === 'undefined')
+    return scheduleMicrotask;
+
+  // In the browser, alternate with setTimeout to avoid freezing
+  let i = 0;
+  return (task: Task) => {
+    if (++i < 100)
+      scheduleMicrotask(task);
+    else
+      setTimeout(task, i = 0);
+  }
+}
+
+export type Task = () => void;
+export type TaskScheduler = (task: Task) => void;

queue-microtask.d.ts

@@ -4,6 +4,8 @@ import {
   setTaskScheduler,
 } from '../asynciterator.mjs';
 
+import createTaskScheduler from '../taskscheduler.mjs';
+
 describe('TaskScheduler', () => {
   describe('scheduleTask', () => {
     it('is a function', () => {
@@ -48,4 +50,51 @@ describe('TaskScheduler', () => {
       setTaskScheduler(scheduler);
     });
   });
+
+  describe('a task scheduler for the browser', () => {
+    const backups = {};
+
+    before(() => {
+      backups.setTimeout = global.setTimeout;
+      backups.queueMicrotask = global.queueMicrotask;
+      global.window = {};
+      global.setTimeout = sinon.spy();
+      global.queueMicrotask = sinon.spy();
+    });
+
+    after(() => {
+      global.setTimeout = backups.setTimeout;
+      global.queueMicrotask = backups.queueMicrotask;
+      delete global.window;
+    });
+
+    it('alternates between setTimeout and queueMicrotask', () => {
+      const taskScheduler = createTaskScheduler();
+      const task = sinon.spy();
+      for (let i = 0; i < 100; i++)
+        taskScheduler(task);
+      expect(global.setTimeout).to.have.callCount(1);
+      expect(global.queueMicrotask).to.have.callCount(99);
+      expect(global.setTimeout).to.have.been.calledWith(task, 0);
+      expect(global.queueMicrotask).to.have.been.calledWith(task);
+    });
+  });
+
+  describe('a task scheduler when queueMicrotask is unavailable', () => {
+    const backups = {};
+
+    before(() => {
+      backups.queueMicrotask = global.queueMicrotask;
+      delete global.queueMicrotask;
+    });
+
+    after(() => {
+      global.queueMicrotask = backups.queueMicrotask;
+    });
+
+    it('can schedule a task', done => {
+      const taskScheduler = createTaskScheduler();
+      taskScheduler(done);
+    });
+  });
 });