diff --git a/.travis.yml b/.travis.yml
index 14344474093..25b7795e13d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -119,7 +119,7 @@ matrix:
 
     - name: Javascript export template (release, emscripten latest)
       stage: build
-      env: PLATFORM=javascript TOOLS=no TARGET=release CACHE_NAME=${PLATFORM}-emcc-latest EXTRA_ARGS="module_glslang_enabled=no"
+      env: PLATFORM=javascript TOOLS=no TARGET=release CACHE_NAME=${PLATFORM}-emcc-latest EXTRA_ARGS="use_closure_compiler=yes"
       os: linux
       compiler: clang
       addons:
diff --git a/platform/javascript/SCsub b/platform/javascript/SCsub
index 85a633442e8..d3cd8f76b75 100644
--- a/platform/javascript/SCsub
+++ b/platform/javascript/SCsub
@@ -10,8 +10,11 @@ javascript_files = [
     'os_javascript.cpp',
 ]
 
-build = env.add_program(['#bin/godot${PROGSUFFIX}.js', '#bin/godot${PROGSUFFIX}.wasm'], javascript_files);
-js, wasm = build
+build_targets = ['#bin/godot${PROGSUFFIX}.js', '#bin/godot${PROGSUFFIX}.wasm']
+if env['threads_enabled']:
+    build_targets.append('#bin/godot${PROGSUFFIX}.worker.js')
+
+build = env.add_program(build_targets, javascript_files)
 
 js_libraries = [
     'http_request.js',
@@ -27,18 +30,38 @@ for module in js_modules:
     env.Append(LINKFLAGS=['--pre-js', env.File(module).path])
 env.Depends(build, js_modules)
 
-wrapper_start = env.File('pre.js')
-wrapper_end = env.File('engine.js')
-js_wrapped = env.Textfile('#bin/godot', [wrapper_start, js, wrapper_end], TEXTFILESUFFIX='${PROGSUFFIX}.wrapped.js')
+engine = [
+    'engine/preloader.js',
+    'engine/loader.js',
+    'engine/utils.js',
+    'engine/engine.js',
+]
+externs = [
+    env.File('#platform/javascript/engine/externs.js')
+]
+js_engine = env.CreateEngineFile('#bin/godot${PROGSUFFIX}.engine.js', engine, externs)
+env.Depends(js_engine, externs)
+
+wrap_list = [
+    build[0],
+    js_engine,
+]
+js_wrapped = env.Textfile('#bin/godot', [env.File(f) for f in wrap_list], TEXTFILESUFFIX='${PROGSUFFIX}.wrapped.js')
 
 zip_dir = env.Dir('#bin/.javascript_zip')
-zip_files = env.InstallAs([
+out_files = [
     zip_dir.File('godot.js'),
     zip_dir.File('godot.wasm'),
     zip_dir.File('godot.html')
-], [
+]
+in_files = [
     js_wrapped,
-    wasm,
+    build[1],
     '#misc/dist/html/full-size.html'
-])
+]
+if env['threads_enabled']:
+    in_files.append(build[2])
+    out_files.append(zip_dir.File('godot.worker.js'))
+
+zip_files = env.InstallAs(out_files, in_files)
 env.Zip('#bin/godot', zip_files, ZIPROOT=zip_dir, ZIPSUFFIX='${PROGSUFFIX}${ZIPSUFFIX}', ZIPCOMSTR='Archving $SOURCES as $TARGET')
diff --git a/platform/javascript/detect.py b/platform/javascript/detect.py
index 17668333647..fb02752aa7e 100644
--- a/platform/javascript/detect.py
+++ b/platform/javascript/detect.py
@@ -1,5 +1,6 @@
 import os
 
+from emscripten_helpers import parse_config, run_closure_compiler, create_engine_file
 
 def is_active():
     return True
@@ -18,6 +19,8 @@ def get_opts():
     return [
         # eval() can be a security concern, so it can be disabled.
         BoolVariable('javascript_eval', 'Enable JavaScript eval interface', True),
+        BoolVariable('threads_enabled', 'Enable WebAssembly Threads support (limited browser support)', False),
+        BoolVariable('use_closure_compiler', 'Use closure compiler to minimize Javascript code', False),
     ]
 
 
@@ -37,7 +40,7 @@ def configure(env):
 
     ## Build type
 
-    if env['target'] != 'debug':
+    if env['target'] == 'release':
         # Use -Os to prioritize optimizing for reduced file size. This is
         # particularly valuable for the web platform because it directly
         # decreases download time.
@@ -46,38 +49,55 @@ def configure(env):
         # run-time performance.
         env.Append(CCFLAGS=['-Os'])
         env.Append(LINKFLAGS=['-Os'])
-        if env['target'] == 'release_debug':
-            env.Append(CPPDEFINES=['DEBUG_ENABLED'])
-            # Retain function names for backtraces at the cost of file size.
-            env.Append(LINKFLAGS=['--profiling-funcs'])
-    else:
+    elif env['target'] == 'release_debug':
+        env.Append(CCFLAGS=['-Os'])
+        env.Append(LINKFLAGS=['-Os'])
+        env.Append(CPPDEFINES=['DEBUG_ENABLED'])
+        # Retain function names for backtraces at the cost of file size.
+        env.Append(LINKFLAGS=['--profiling-funcs'])
+    else: # 'debug'
         env.Append(CPPDEFINES=['DEBUG_ENABLED'])
         env.Append(CCFLAGS=['-O1', '-g'])
         env.Append(LINKFLAGS=['-O1', '-g'])
         env.Append(LINKFLAGS=['-s', 'ASSERTIONS=1'])
 
-    ## Compiler configuration
+    if env['tools']:
+        if not env['threads_enabled']:
+            raise RuntimeError("Threads must be enabled to build the editor. Please add the 'threads_enabled=yes' option")
+        # Tools need more memory. Initial stack memory in bytes. See `src/settings.js` in emscripten repository (will be renamed to INITIAL_MEMORY).
+        env.Append(LINKFLAGS=['-s', 'TOTAL_MEMORY=33554432'])
+    else:
+        # Disable exceptions and rtti on non-tools (template) builds
+        # These flags help keep the file size down.
+        env.Append(CCFLAGS=['-fno-exceptions', '-fno-rtti'])
+        # Don't use dynamic_cast, necessary with no-rtti.
+        env.Append(CPPDEFINES=['NO_SAFE_CAST'])
 
+    ## Copy env variables.
     env['ENV'] = os.environ
 
-    em_config_file = os.getenv('EM_CONFIG') or os.path.expanduser('~/.emscripten')
-    if not os.path.exists(em_config_file):
-        raise RuntimeError("Emscripten configuration file '%s' does not exist" % em_config_file)
-    with open(em_config_file) as f:
-        em_config = {}
-        try:
-            # Emscripten configuration file is a Python file with simple assignments.
-            exec(f.read(), em_config)
-        except StandardError as e:
-            raise RuntimeError("Emscripten configuration file '%s' is invalid:\n%s" % (em_config_file, e))
-    if 'BINARYEN_ROOT' in em_config and os.path.isdir(os.path.join(em_config.get('BINARYEN_ROOT'), 'emscripten')):
-        # New style, emscripten path as a subfolder of BINARYEN_ROOT
-        env.PrependENVPath('PATH', os.path.join(em_config.get('BINARYEN_ROOT'), 'emscripten'))
-    elif 'EMSCRIPTEN_ROOT' in em_config:
-        # Old style (but can be there as a result from previous activation, so do last)
-        env.PrependENVPath('PATH', em_config.get('EMSCRIPTEN_ROOT'))
-    else:
-        raise RuntimeError("'BINARYEN_ROOT' or 'EMSCRIPTEN_ROOT' missing in Emscripten configuration file '%s'" % em_config_file)
+    # LTO
+    if env['use_lto']:
+        env.Append(CCFLAGS=['-s', 'WASM_OBJECT_FILES=0'])
+        env.Append(LINKFLAGS=['-s', 'WASM_OBJECT_FILES=0'])
+        env.Append(LINKFLAGS=['--llvm-lto',  '1'])
+
+    # Closure compiler
+    if env['use_closure_compiler']:
+        # For emscripten support code.
+        env.Append(LINKFLAGS=['--closure', '1'])
+        # Register builder for our Engine files
+        jscc = env.Builder(generator=run_closure_compiler, suffix='.cc.js', src_suffix='.js')
+        env.Append(BUILDERS = {'BuildJS' : jscc})
+
+    # Add method that joins/compiles our Engine files.
+    env.AddMethod(create_engine_file, "CreateEngineFile")
+
+    # Closure compiler extern and support for ecmascript specs (const, let, etc).
+    env['ENV']['EMCC_CLOSURE_ARGS'] = '--language_in ECMASCRIPT6'
+
+    em_config = parse_config()
+    env.PrependENVPath('PATH', em_config['EMCC_ROOT'])
 
     env['CC'] = 'emcc'
     env['CXX'] = 'em++'
@@ -104,44 +124,31 @@ def configure(env):
     env['LIBPREFIXES'] = ['$LIBPREFIX']
     env['LIBSUFFIXES'] = ['$LIBSUFFIX']
 
-    ## Compile flags
-
     env.Prepend(CPPPATH=['#platform/javascript'])
     env.Append(CPPDEFINES=['JAVASCRIPT_ENABLED', 'UNIX_ENABLED'])
 
-    # No multi-threading (SharedArrayBuffer) available yet,
-    # once feasible also consider memory buffer size issues.
-    env.Append(CPPDEFINES=['NO_THREADS'])
-
-    # Disable exceptions and rtti on non-tools (template) builds
-    if not env['tools']:
-        # These flags help keep the file size down.
-        env.Append(CCFLAGS=['-fno-exceptions', '-fno-rtti'])
-        # Don't use dynamic_cast, necessary with no-rtti.
-        env.Append(CPPDEFINES=['NO_SAFE_CAST'])
-
     if env['javascript_eval']:
         env.Append(CPPDEFINES=['JAVASCRIPT_EVAL_ENABLED'])
 
-    ## Link flags
+    # Thread support (via SharedArrayBuffer).
+    if env['threads_enabled']:
+        env.Append(CPPDEFINES=['PTHREAD_NO_RENAME'])
+        env.Append(CCFLAGS=['-s', 'USE_PTHREADS=1'])
+        env.Append(LINKFLAGS=['-s', 'USE_PTHREADS=1'])
+        env.Append(LINKFLAGS=['-s', 'PTHREAD_POOL_SIZE=4'])
+        env.Append(LINKFLAGS=['-s', 'WASM_MEM_MAX=2048MB'])
+    else:
+        env.Append(CPPDEFINES=['NO_THREADS'])
+
+    # Reduce code size by generating less support code (e.g. skip NodeJS support).
+    env.Append(LINKFLAGS=['-s', 'ENVIRONMENT=web,worker'])
 
     # We use IDBFS in javascript_main.cpp. Since Emscripten 1.39.1 it needs to
     # be linked explicitly.
     env.Append(LIBS=['idbfs.js'])
 
     env.Append(LINKFLAGS=['-s', 'BINARYEN=1'])
-
-    # Only include the JavaScript support code for the web environment
-    # (i.e. exclude Node.js and other unused environments).
-    # This makes the JavaScript support code about 4 KB smaller.
-    env.Append(LINKFLAGS=['-s', 'ENVIRONMENT=web'])
-
-    # This needs to be defined for Emscripten using 'fastcomp' (default pre-1.39.0)
-    # and undefined if using 'upstream'. And to make things simple, earlier
-    # Emscripten versions didn't include 'fastcomp' in their path, so we check
-    # against the presence of 'upstream' to conditionally add the flag.
-    if not "upstream" in em_config['EMSCRIPTEN_ROOT']:
-        env.Append(LINKFLAGS=['-s', 'BINARYEN_TRAP_MODE=\'clamp\''])
+    env.Append(LINKFLAGS=['-s', 'MODULARIZE=1', '-s', 'EXPORT_NAME="Godot"'])
 
     # Allow increasing memory buffer size during runtime. This is efficient
     # when using WebAssembly (in comparison to asm.js) and works well for
@@ -153,8 +160,5 @@ def configure(env):
 
     env.Append(LINKFLAGS=['-s', 'INVOKE_RUN=0'])
 
-    # TODO: Reevaluate usage of this setting now that engine.js manages engine runtime.
-    env.Append(LINKFLAGS=['-s', 'NO_EXIT_RUNTIME=1'])
-
-    #adding flag due to issue with emscripten 1.38.41 callMain method https://github.com/emscripten-core/emscripten/blob/incoming/ChangeLog.md#v13841-08072019
-    env.Append(LINKFLAGS=['-s', 'EXTRA_EXPORTED_RUNTIME_METHODS=["callMain"]'])
+    # callMain for manual start, FS for preloading.
+    env.Append(LINKFLAGS=['-s', 'EXTRA_EXPORTED_RUNTIME_METHODS=["callMain", "FS"]'])
diff --git a/platform/javascript/emscripten_helpers.py b/platform/javascript/emscripten_helpers.py
new file mode 100644
index 00000000000..bda5b40a74c
--- /dev/null
+++ b/platform/javascript/emscripten_helpers.py
@@ -0,0 +1,37 @@
+import os
+
+def parse_config():
+    em_config_file = os.getenv('EM_CONFIG') or os.path.expanduser('~/.emscripten')
+    if not os.path.exists(em_config_file):
+        raise RuntimeError("Emscripten configuration file '%s' does not exist" % em_config_file)
+
+    normalized = {}
+    em_config = {}
+    with open(em_config_file) as f:
+        try:
+            # Emscripten configuration file is a Python file with simple assignments.
+            exec(f.read(), em_config)
+        except StandardError as e:
+            raise RuntimeError("Emscripten configuration file '%s' is invalid:\n%s" % (em_config_file, e))
+    normalized['EMCC_ROOT'] = em_config.get('EMSCRIPTEN_ROOT')
+    normalized['NODE_JS'] = em_config.get('NODE_JS')
+    normalized['CLOSURE_BIN'] = os.path.join(normalized['EMCC_ROOT'], 'node_modules', '.bin', 'google-closure-compiler')
+    return normalized
+
+
+def run_closure_compiler(target, source, env, for_signature):
+    cfg = parse_config()
+    cmd = [cfg['NODE_JS'], cfg['CLOSURE_BIN']]
+    cmd.extend(['--compilation_level', 'ADVANCED_OPTIMIZATIONS'])
+    for f in env['JSEXTERNS']:
+        cmd.extend(['--externs', f.get_abspath()])
+    for f in source:
+        cmd.extend(['--js', f.get_abspath()])
+    cmd.extend(['--js_output_file', target[0].get_abspath()])
+    return ' '.join(cmd)
+
+
+def create_engine_file(env, target, source, externs):
+    if env['use_closure_compiler']:
+        return env.BuildJS(target, source, JSEXTERNS=externs)
+    return env.Textfile(target, [env.File(s) for s in source])
diff --git a/platform/javascript/engine.js b/platform/javascript/engine.js
deleted file mode 100644
index 227accadb0e..00000000000
--- a/platform/javascript/engine.js
+++ /dev/null
@@ -1,411 +0,0 @@
-		// The following is concatenated with generated code, and acts as the end
-		// of a wrapper for said code. See pre.js for the other part of the
-		// wrapper.
-		exposedLibs['PATH'] = PATH;
-		exposedLibs['FS'] = FS;
-		return Module;
-	},
-};
-
-(function() {
-	var engine = Engine;
-
-	var DOWNLOAD_ATTEMPTS_MAX = 4;
-
-	var basePath = null;
-	var wasmFilenameExtensionOverride = null;
-	var engineLoadPromise = null;
-
-	var loadingFiles = {};
-
-	function getPathLeaf(path) {
-
-		while (path.endsWith('/'))
-			path = path.slice(0, -1);
-		return path.slice(path.lastIndexOf('/') + 1);
-	}
-
-	function getBasePath(path) {
-
-		if (path.endsWith('/'))
-			path = path.slice(0, -1);
-		if (path.lastIndexOf('.') > path.lastIndexOf('/'))
-			path = path.slice(0, path.lastIndexOf('.'));
-		return path;
-	}
-
-	function getBaseName(path) {
-
-		return getPathLeaf(getBasePath(path));
-	}
-
-	Engine = function Engine() {
-
-		this.rtenv = null;
-
-		var LIBS = {};
-
-		var initPromise = null;
-		var unloadAfterInit = true;
-
-		var preloadedFiles = [];
-
-		var resizeCanvasOnStart = true;
-		var progressFunc = null;
-		var preloadProgressTracker = {};
-		var lastProgress = { loaded: 0, total: 0 };
-
-		var canvas = null;
-		var executableName = null;
-		var locale = null;
-		var stdout = null;
-		var stderr = null;
-
-		this.init = function(newBasePath) {
-
-			if (!initPromise) {
-				initPromise = Engine.load(newBasePath).then(
-					instantiate.bind(this)
-				);
-				requestAnimationFrame(animateProgress);
-				if (unloadAfterInit)
-					initPromise.then(Engine.unloadEngine);
-			}
-			return initPromise;
-		};
-
-		function instantiate(wasmBuf) {
-
-			var rtenvProps = {
-				engine: this,
-				ENV: {},
-			};
-			if (typeof stdout === 'function')
-				rtenvProps.print = stdout;
-			if (typeof stderr === 'function')
-				rtenvProps.printErr = stderr;
-			rtenvProps.instantiateWasm = function(imports, onSuccess) {
-				WebAssembly.instantiate(wasmBuf, imports).then(function(result) {
-					onSuccess(result.instance);
-				});
-				return {};
-			};
-
-			return new Promise(function(resolve, reject) {
-				rtenvProps.onRuntimeInitialized = resolve;
-				rtenvProps.onAbort = reject;
-				rtenvProps.thisProgram = executableName;
-				rtenvProps.engine.rtenv = Engine.RuntimeEnvironment(rtenvProps, LIBS);
-			});
-		}
-
-		this.preloadFile = function(pathOrBuffer, destPath) {
-
-			if (pathOrBuffer instanceof ArrayBuffer) {
-				pathOrBuffer = new Uint8Array(pathOrBuffer);
-			} else if (ArrayBuffer.isView(pathOrBuffer)) {
-				pathOrBuffer = new Uint8Array(pathOrBuffer.buffer);
-			}
-			if (pathOrBuffer instanceof Uint8Array) {
-				preloadedFiles.push({
-					path: destPath,
-					buffer: pathOrBuffer
-				});
-				return Promise.resolve();
-			} else if (typeof pathOrBuffer === 'string') {
-				return loadPromise(pathOrBuffer, preloadProgressTracker).then(function(xhr) {
-					preloadedFiles.push({
-						path: destPath || pathOrBuffer,
-						buffer: xhr.response
-					});
-				});
-			} else {
-				throw Promise.reject("Invalid object for preloading");
-			}
-		};
-
-		this.start = function() {
-
-			return this.init().then(
-				Function.prototype.apply.bind(synchronousStart, this, arguments)
-			);
-		};
-
-		this.startGame = function(execName, mainPack) {
-
-			executableName = execName;
-			var mainArgs = [ '--main-pack', getPathLeaf(mainPack) ];
-
-			return Promise.all([
-				this.init(getBasePath(execName)),
-				this.preloadFile(mainPack, getPathLeaf(mainPack))
-			]).then(
-				Function.prototype.apply.bind(synchronousStart, this, mainArgs)
-			);
-		};
-
-		function synchronousStart() {
-
-			if (canvas instanceof HTMLCanvasElement) {
-				this.rtenv.canvas = canvas;
-			} else {
-				var firstCanvas = document.getElementsByTagName('canvas')[0];
-				if (firstCanvas instanceof HTMLCanvasElement) {
-					this.rtenv.canvas = firstCanvas;
-				} else {
-					throw new Error("No canvas found");
-				}
-			}
-
-			var actualCanvas = this.rtenv.canvas;
-			// canvas can grab focus on click
-			if (actualCanvas.tabIndex < 0) {
-				actualCanvas.tabIndex = 0;
-			}
-			// necessary to calculate cursor coordinates correctly
-			actualCanvas.style.padding = 0;
-			actualCanvas.style.borderWidth = 0;
-			actualCanvas.style.borderStyle = 'none';
-			// disable right-click context menu
-			actualCanvas.addEventListener('contextmenu', function(ev) {
-				ev.preventDefault();
-			}, false);
-			// until context restoration is implemented
-			actualCanvas.addEventListener('webglcontextlost', function(ev) {
-				alert("WebGL context lost, please reload the page");
-				ev.preventDefault();
-			}, false);
-
-			if (locale) {
-				this.rtenv.locale = locale;
-			} else {
-				this.rtenv.locale = navigator.languages ? navigator.languages[0] : navigator.language;
-			}
-			this.rtenv.locale = this.rtenv.locale.split('.')[0];
-			this.rtenv.resizeCanvasOnStart = resizeCanvasOnStart;
-
-			preloadedFiles.forEach(function(file) {
-				var dir = LIBS.PATH.dirname(file.path);
-				try {
-					LIBS.FS.stat(dir);
-				} catch (e) {
-					if (e.code !== 'ENOENT') {
-						throw e;
-					}
-					LIBS.FS.mkdirTree(dir);
-				}
-				// With memory growth, canOwn should be false.
-				LIBS.FS.createDataFile(file.path, null, new Uint8Array(file.buffer), true, true, false);
-			}, this);
-
-			preloadedFiles = null;
-			initPromise = null;
-			this.rtenv.callMain(arguments);
-		}
-
-		this.setProgressFunc = function(func) {
-			progressFunc = func;
-		};
-
-		this.setResizeCanvasOnStart = function(enabled) {
-			resizeCanvasOnStart = enabled;
-		};
-
-		function animateProgress() {
-
-			var loaded = 0;
-			var total = 0;
-			var totalIsValid = true;
-			var progressIsFinal = true;
-
-			[loadingFiles, preloadProgressTracker].forEach(function(tracker) {
-				Object.keys(tracker).forEach(function(file) {
-					if (!tracker[file].final)
-						progressIsFinal = false;
-					if (!totalIsValid || tracker[file].total === 0) {
-						totalIsValid = false;
-						total = 0;
-					} else {
-						total += tracker[file].total;
-					}
-					loaded += tracker[file].loaded;
-				});
-			});
-			if (loaded !== lastProgress.loaded || total !== lastProgress.total) {
-				lastProgress.loaded = loaded;
-				lastProgress.total = total;
-				if (typeof progressFunc === 'function')
-					progressFunc(loaded, total);
-			}
-			if (!progressIsFinal)
-				requestAnimationFrame(animateProgress);
-		}
-
-		this.setCanvas = function(elem) {
-			canvas = elem;
-		};
-
-		this.setExecutableName = function(newName) {
-
-			executableName = newName;
-		};
-
-		this.setLocale = function(newLocale) {
-
-			locale = newLocale;
-		};
-
-		this.setUnloadAfterInit = function(enabled) {
-
-			if (enabled && !unloadAfterInit && initPromise) {
-				initPromise.then(Engine.unloadEngine);
-			}
-			unloadAfterInit = enabled;
-		};
-
-		this.setStdoutFunc = function(func) {
-
-			var print = function(text) {
-				if (arguments.length > 1) {
-					text = Array.prototype.slice.call(arguments).join(" ");
-				}
-				func(text);
-			};
-			if (this.rtenv)
-				this.rtenv.print = print;
-			stdout = print;
-		};
-
-		this.setStderrFunc = function(func) {
-
-			var printErr = function(text) {
-				if (arguments.length > 1)
-					text = Array.prototype.slice.call(arguments).join(" ");
-				func(text);
-			};
-			if (this.rtenv)
-				this.rtenv.printErr = printErr;
-			stderr = printErr;
-		};
-
-
-	}; // Engine()
-
-	Engine.RuntimeEnvironment = engine.RuntimeEnvironment;
-
-	Engine.isWebGLAvailable = function(majorVersion = 1) {
-
-		var testContext = false;
-		try {
-			var testCanvas = document.createElement('canvas');
-			if (majorVersion === 1) {
-				testContext = testCanvas.getContext('webgl') || testCanvas.getContext('experimental-webgl');
-			} else if (majorVersion === 2) {
-				testContext = testCanvas.getContext('webgl2') || testCanvas.getContext('experimental-webgl2');
-			}
-		} catch (e) {}
-		return !!testContext;
-	};
-
-	Engine.setWebAssemblyFilenameExtension = function(override) {
-
-		if (String(override).length === 0) {
-			throw new Error('Invalid WebAssembly filename extension override');
-		}
-		wasmFilenameExtensionOverride = String(override);
-	}
-
-	Engine.load = function(newBasePath) {
-
-		if (newBasePath !== undefined) basePath = getBasePath(newBasePath);
-		if (engineLoadPromise === null) {
-			if (typeof WebAssembly !== 'object')
-				return Promise.reject(new Error("Browser doesn't support WebAssembly"));
-			// TODO cache/retrieve module to/from idb
-			engineLoadPromise = loadPromise(basePath + '.' + (wasmFilenameExtensionOverride || 'wasm')).then(function(xhr) {
-				return xhr.response;
-			});
-			engineLoadPromise = engineLoadPromise.catch(function(err) {
-				engineLoadPromise = null;
-				throw err;
-			});
-		}
-		return engineLoadPromise;
-	};
-
-	Engine.unload = function() {
-		engineLoadPromise = null;
-	};
-
-	function loadPromise(file, tracker) {
-		if (tracker === undefined)
-			tracker = loadingFiles;
-		return new Promise(function(resolve, reject) {
-			loadXHR(resolve, reject, file, tracker);
-		});
-	}
-
-	function loadXHR(resolve, reject, file, tracker) {
-
-		var xhr = new XMLHttpRequest;
-		xhr.open('GET', file);
-		if (!file.endsWith('.js')) {
-			xhr.responseType = 'arraybuffer';
-		}
-		['loadstart', 'progress', 'load', 'error', 'abort'].forEach(function(ev) {
-			xhr.addEventListener(ev, onXHREvent.bind(xhr, resolve, reject, file, tracker));
-		});
-		xhr.send();
-	}
-
-	function onXHREvent(resolve, reject, file, tracker, ev) {
-
-		if (this.status >= 400) {
-
-			if (this.status < 500 || ++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
-				reject(new Error("Failed loading file '" + file + "': " + this.statusText));
-				this.abort();
-				return;
-			} else {
-				setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
-			}
-		}
-
-		switch (ev.type) {
-			case 'loadstart':
-				if (tracker[file] === undefined) {
-					tracker[file] = {
-						total: ev.total,
-						loaded: ev.loaded,
-						attempts: 0,
-						final: false,
-					};
-				}
-				break;
-
-			case 'progress':
-				tracker[file].loaded = ev.loaded;
-				tracker[file].total = ev.total;
-				break;
-
-			case 'load':
-				tracker[file].final = true;
-				resolve(this);
-				break;
-
-			case 'error':
-				if (++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
-					tracker[file].final = true;
-					reject(new Error("Failed loading file '" + file + "'"));
-				} else {
-					setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
-				}
-				break;
-
-			case 'abort':
-				tracker[file].final = true;
-				reject(new Error("Loading file '" + file + "' was aborted."));
-				break;
-		}
-	}
-})();
diff --git a/platform/javascript/engine/engine.js b/platform/javascript/engine/engine.js
new file mode 100644
index 00000000000..6d7509377fc
--- /dev/null
+++ b/platform/javascript/engine/engine.js
@@ -0,0 +1,184 @@
+Function('return this')()['Engine'] = (function() {
+
+	var unloadAfterInit = true;
+	var canvas = null;
+	var resizeCanvasOnStart = false;
+	var customLocale = 'en_US';
+	var wasmExt = '.wasm';
+
+	var preloader = new Preloader();
+	var loader = new Loader();
+	var rtenv = null;
+
+	var executableName = '';
+	var loadPath = '';
+	var loadPromise = null;
+	var initPromise = null;
+	var stderr = null;
+	var stdout = null;
+	var progressFunc = null;
+
+	function load(basePath) {
+		if (loadPromise == null) {
+			loadPath = basePath;
+			loadPromise = preloader.loadPromise(basePath + wasmExt);
+			preloader.setProgressFunc(progressFunc);
+			requestAnimationFrame(preloader.animateProgress);
+		}
+		return loadPromise;
+	};
+
+	function unload() {
+		loadPromise = null;
+	};
+
+	/** @constructor */
+	function Engine() {};
+
+	Engine.prototype.init = /** @param {string=} basePath */ function(basePath) {
+		if (initPromise) {
+			return initPromise;
+		}
+		if (!loadPromise) {
+			if (!basePath) {
+				initPromise = Promise.reject(new Error("A base path must be provided when calling `init` and the engine is not loaded."));
+				return initPromise;
+			}
+			load(basePath);
+		}
+		var config = {}
+		if (typeof stdout === 'function')
+			config.print = stdout;
+		if (typeof stderr === 'function')
+			config.printErr = stderr;
+		initPromise = loader.init(loadPromise, loadPath, config).then(function() {
+			return new Promise(function(resolve, reject) {
+				rtenv = loader.env;
+				if (unloadAfterInit) {
+					loadPromise = null;
+				}
+				resolve();
+			});
+		});
+		return initPromise;
+	};
+
+	/** @type {function(string, string):Object} */
+	Engine.prototype.preloadFile = function(file, path) {
+		return preloader.preload(file, path);
+	};
+
+	/** @type {function(...string):Object} */
+	Engine.prototype.start = function() {
+		// Start from arguments.
+		var args = [];
+		for (var i = 0; i < arguments.length; i++) {
+			args.push(arguments[i]);
+		}
+		var me = this;
+		return new Promise(function(resolve, reject) {
+			return me.init().then(function() {
+				if (!(canvas instanceof HTMLCanvasElement)) {
+					canvas = Utils.findCanvas();
+				}
+				rtenv['locale'] = customLocale;
+				rtenv['canvas'] = canvas;
+				rtenv['thisProgram'] = executableName;
+				rtenv['resizeCanvasOnStart'] = resizeCanvasOnStart;
+				loader.start(preloader.preloadedFiles, args).then(function() {
+					loader = null;
+					initPromise = null;
+					resolve();
+				});
+			});
+		});
+	};
+
+	Engine.prototype.startGame = function(execName, mainPack) {
+		// Start and init with execName as loadPath if not inited.
+		executableName = execName;
+		var me = this;
+		return Promise.all([
+			this.init(execName),
+			this.preloadFile(mainPack, mainPack)
+		]).then(function() {
+			return me.start('--main-pack', mainPack);
+		});
+	};
+
+	Engine.prototype.setWebAssemblyFilenameExtension = function(override) {
+		if (String(override).length === 0) {
+			throw new Error('Invalid WebAssembly filename extension override');
+		}
+		wasmExt = String(override);
+	};
+
+	Engine.prototype.setUnloadAfterInit = function(enabled) {
+		unloadAfterInit = enabled;
+	};
+
+	Engine.prototype.setCanvas = function(canvasElem) {
+		canvas = canvasElem;
+	};
+
+	Engine.prototype.setCanvasResizedOnStart = function(enabled) {
+		resizeCanvasOnStart = enabled;
+	};
+
+	Engine.prototype.setLocale = function(locale) {
+		customLocale = locale;
+	};
+
+	Engine.prototype.setExecutableName = function(newName) {
+		executableName = newName;
+	};
+
+	Engine.prototype.setProgressFunc = function(func) {
+		progressFunc = func;
+	}
+
+	Engine.prototype.setStdoutFunc = function(func) {
+
+		var print = function(text) {
+			if (arguments.length > 1) {
+				text = Array.prototype.slice.call(arguments).join(" ");
+			}
+			func(text);
+		};
+		if (rtenv)
+			rtenv.print = print;
+		stdout = print;
+	};
+
+	Engine.prototype.setStderrFunc = function(func) {
+
+		var printErr = function(text) {
+			if (arguments.length > 1)
+				text = Array.prototype.slice.call(arguments).join(" ");
+			func(text);
+		};
+		if (rtenv)
+			rtenv.printErr = printErr;
+		stderr = printErr;
+	};
+
+	// Closure compiler exported engine methods.
+	/** @export */
+	Engine['isWebGLAvailable'] = Utils.isWebGLAvailable;
+	Engine['load'] = load;
+	Engine['unload'] = unload;
+	Engine.prototype['init'] = Engine.prototype.init
+	Engine.prototype['preloadFile'] = Engine.prototype.preloadFile
+	Engine.prototype['start'] = Engine.prototype.start
+	Engine.prototype['startGame'] = Engine.prototype.startGame
+	Engine.prototype['setWebAssemblyFilenameExtension'] = Engine.prototype.setWebAssemblyFilenameExtension
+	Engine.prototype['setUnloadAfterInit'] = Engine.prototype.setUnloadAfterInit
+	Engine.prototype['setCanvas'] = Engine.prototype.setCanvas
+	Engine.prototype['setCanvasResizedOnStart'] = Engine.prototype.setCanvasResizedOnStart
+	Engine.prototype['setLocale'] = Engine.prototype.setLocale
+	Engine.prototype['setExecutableName'] = Engine.prototype.setExecutableName
+	Engine.prototype['setProgressFunc'] = Engine.prototype.setProgressFunc
+	Engine.prototype['setStdoutFunc'] = Engine.prototype.setStdoutFunc
+	Engine.prototype['setStderrFunc'] = Engine.prototype.setStderrFunc
+	return Engine;
+})();
diff --git a/platform/javascript/engine/externs.js b/platform/javascript/engine/externs.js
new file mode 100644
index 00000000000..1a94dd15ec6
--- /dev/null
+++ b/platform/javascript/engine/externs.js
@@ -0,0 +1,3 @@
+var Godot;
+var WebAssembly = {};
+WebAssembly.instantiate = function(buffer, imports) {};
diff --git a/platform/javascript/engine/loader.js b/platform/javascript/engine/loader.js
new file mode 100644
index 00000000000..d27fbf612ee
--- /dev/null
+++ b/platform/javascript/engine/loader.js
@@ -0,0 +1,33 @@
+var Loader = /** @constructor */ function() {
+
+	this.env = null;
+
+	this.init = function(loadPromise, basePath, config) {
+		var me = this;
+		return new Promise(function(resolve, reject) {
+			var cfg = config || {};
+			cfg['locateFile'] = Utils.createLocateRewrite(basePath);
+			cfg['instantiateWasm'] = Utils.createInstantiatePromise(loadPromise);
+			loadPromise = null;
+			Godot(cfg).then(function(module) {
+				me.env = module;
+				resolve();
+			});
+		});
+	}
+
+	this.start = function(preloadedFiles, args) {
+		var me = this;
+		return new Promise(function(resolve, reject) {
+			if (!me.env) {
+				reject(new Error('The engine must be initialized before it can be started'));
+			}
+			preloadedFiles.forEach(function(file) {
+				Utils.copyToFS(me.env['FS'], file.path, file.buffer);
+			});
+			preloadedFiles.length = 0; // Clear memory
+			me.env['callMain'](args);
+			resolve();
+		});
+	}
+};
diff --git a/platform/javascript/engine/preloader.js b/platform/javascript/engine/preloader.js
new file mode 100644
index 00000000000..17918eae382
--- /dev/null
+++ b/platform/javascript/engine/preloader.js
@@ -0,0 +1,139 @@
+var Preloader = /** @constructor */ function() {
+
+	var DOWNLOAD_ATTEMPTS_MAX = 4;
+	var progressFunc = null;
+	var lastProgress = { loaded: 0, total: 0 };
+
+	var loadingFiles = {};
+	this.preloadedFiles = [];
+
+	function loadXHR(resolve, reject, file, tracker) {
+		var xhr = new XMLHttpRequest;
+		xhr.open('GET', file);
+		if (!file.endsWith('.js')) {
+			xhr.responseType = 'arraybuffer';
+		}
+		['loadstart', 'progress', 'load', 'error', 'abort'].forEach(function(ev) {
+			xhr.addEventListener(ev, onXHREvent.bind(xhr, resolve, reject, file, tracker));
+		});
+		xhr.send();
+	}
+
+	function onXHREvent(resolve, reject, file, tracker, ev) {
+
+		if (this.status >= 400) {
+
+			if (this.status < 500 || ++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
+				reject(new Error("Failed loading file '" + file + "': " + this.statusText));
+				this.abort();
+				return;
+			} else {
+				setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
+			}
+		}
+
+		switch (ev.type) {
+			case 'loadstart':
+				if (tracker[file] === undefined) {
+					tracker[file] = {
+						total: ev.total,
+						loaded: ev.loaded,
+						attempts: 0,
+						final: false,
+					};
+				}
+				break;
+
+			case 'progress':
+				tracker[file].loaded = ev.loaded;
+				tracker[file].total = ev.total;
+				break;
+
+			case 'load':
+				tracker[file].final = true;
+				resolve(this);
+				break;
+
+			case 'error':
+				if (++tracker[file].attempts >= DOWNLOAD_ATTEMPTS_MAX) {
+					tracker[file].final = true;
+					reject(new Error("Failed loading file '" + file + "'"));
+				} else {
+					setTimeout(loadXHR.bind(null, resolve, reject, file, tracker), 1000);
+				}
+				break;
+
+			case 'abort':
+				tracker[file].final = true;
+				reject(new Error("Loading file '" + file + "' was aborted."));
+				break;
+		}
+	}
+
+	this.loadPromise = function(file) {
+		return new Promise(function(resolve, reject) {
+			loadXHR(resolve, reject, file, loadingFiles);
+		});
+	}
+
+	this.preload = function(pathOrBuffer, destPath) {
+		if (pathOrBuffer instanceof ArrayBuffer) {
+			pathOrBuffer = new Uint8Array(pathOrBuffer);
+		} else if (ArrayBuffer.isView(pathOrBuffer)) {
+			pathOrBuffer = new Uint8Array(pathOrBuffer.buffer);
+		}
+		if (pathOrBuffer instanceof Uint8Array) {
+			this.preloadedFiles.push({
+				path: destPath,
+				buffer: pathOrBuffer
+			});
+			return Promise.resolve();
+		} else if (typeof pathOrBuffer === 'string') {
+			var me = this;
+			return this.loadPromise(pathOrBuffer).then(function(xhr) {
+				me.preloadedFiles.push({
+					path: destPath || pathOrBuffer,
+					buffer: xhr.response
+				});
+				return Promise.resolve();
+			});
+		} else {
+			throw Promise.reject("Invalid object for preloading");
+		}
+	};
+
+	var animateProgress = function() {
+
+		var loaded = 0;
+		var total = 0;
+		var totalIsValid = true;
+		var progressIsFinal = true;
+
+		Object.keys(loadingFiles).forEach(function(file) {
+			const stat = loadingFiles[file];
+			if (!stat.final) {
+				progressIsFinal = false;
+			}
+			if (!totalIsValid || stat.total === 0) {
+				totalIsValid = false;
+				total = 0;
+			} else {
+				total += stat.total;
+			}
+			loaded += stat.loaded;
+		});
+		if (loaded !== lastProgress.loaded || total !== lastProgress.total) {
+			lastProgress.loaded = loaded;
+			lastProgress.total = total;
+			if (typeof progressFunc === 'function')
+				progressFunc(loaded, total);
+		}
+		if (!progressIsFinal)
+			requestAnimationFrame(animateProgress);
+	}
+	this.animateProgress = animateProgress; // Also exposed to start it.
+
+	this.setProgressFunc = function(callback) {
+		progressFunc = callback;
+	}
+};
diff --git a/platform/javascript/engine/utils.js b/platform/javascript/engine/utils.js
new file mode 100644
index 00000000000..fdff90a9236
--- /dev/null
+++ b/platform/javascript/engine/utils.js
@@ -0,0 +1,69 @@
+var Utils = {
+
+	createLocateRewrite: function(execName) {
+		function rw(path) {
+			if (path.endsWith('.worker.js')) {
+				return execName + '.worker.js';
+			} else if (path.endsWith('.js')) {
+				return execName + '.js';
+			} else if (path.endsWith('.wasm')) {
+				return execName + '.wasm';
+			}
+		}
+		return rw;
+	},
+
+	createInstantiatePromise: function(wasmLoader) {
+		function instantiateWasm(imports, onSuccess) {
+			wasmLoader.then(function(xhr) {
+				WebAssembly.instantiate(xhr.response, imports).then(function(result) {
+					onSuccess(result['instance'], result['module']);
+				});
+			});
+			wasmLoader = null;
+			return {};
+		};
+
+		return instantiateWasm;
+	},
+
+	copyToFS: function(fs, path, buffer) {
+		var p = path.lastIndexOf("/");
+		var dir = "/";
+		if (p > 0) {
+			dir = path.slice(0, path.lastIndexOf("/"));
+		}
+		try {
+			fs.stat(dir);
+		} catch (e) {
+			if (e.errno !== 44) { // 'ENOENT', see https://github.com/emscripten-core/emscripten/blob/master/system/lib/libc/musl/arch/emscripten/bits/errno.h
+				throw e;
+			}
+			fs['mkdirTree'](dir);
+		}
+		// With memory growth, canOwn should be false.
+		fs['writeFile'](path, new Uint8Array(buffer), {'flags': 'wx+'});
+	},
+
+	findCanvas: function() {
+		var nodes = document.getElementsByTagName('canvas');
+		if (nodes.length && nodes[0] instanceof HTMLCanvasElement) {
+			return nodes[0];
+		}
+		throw new Error("No canvas found");
+	},
+
+	isWebGLAvailable: function(majorVersion = 1) {
+
+		var testContext = false;
+		try {
+			var testCanvas = document.createElement('canvas');
+			if (majorVersion === 1) {
+				testContext = testCanvas.getContext('webgl') || testCanvas.getContext('experimental-webgl');
+			} else if (majorVersion === 2) {
+				testContext = testCanvas.getContext('webgl2') || testCanvas.getContext('experimental-webgl2');
+			}
+		} catch (e) {}
+		return !!testContext;
+	}
+};
diff --git a/platform/javascript/export/export.cpp b/platform/javascript/export/export.cpp
index f0326d50271..da61425747e 100644
--- a/platform/javascript/export/export.cpp
+++ b/platform/javascript/export/export.cpp
@@ -94,6 +94,9 @@ public:
 		} else if (req[1] == basereq + ".js") {
 			filepath += ".js";
 			ctype = "application/javascript";
+		} else if (req[1] == basereq + ".worker.js") {
+			filepath += ".worker.js";
+			ctype = "application/javascript";
 		} else if (req[1] == basereq + ".pck") {
 			filepath += ".pck";
 			ctype = "application/octet-stream";
@@ -432,6 +435,10 @@ Error EditorExportPlatformJavaScript::export_project(const Ref<EditorExportPrese
 		} else if (file == "godot.js") {
 
 			file = p_path.get_file().get_basename() + ".js";
+		} else if (file == "godot.worker.js") {
+
+			file = p_path.get_file().get_basename() + ".worker.js";
+
 		} else if (file == "godot.wasm") {
 
 			file = p_path.get_file().get_basename() + ".wasm";
@@ -563,6 +570,7 @@ Error EditorExportPlatformJavaScript::run(const Ref<EditorExportPreset> &p_prese
 		// Export generates several files, clean them up on failure.
 		DirAccess::remove_file_or_error(basepath + ".html");
 		DirAccess::remove_file_or_error(basepath + ".js");
+		DirAccess::remove_file_or_error(basepath + ".worker.js");
 		DirAccess::remove_file_or_error(basepath + ".pck");
 		DirAccess::remove_file_or_error(basepath + ".png");
 		DirAccess::remove_file_or_error(basepath + ".wasm");
diff --git a/platform/javascript/id_handler.js b/platform/javascript/id_handler.js
index 3851123ed14..67d29075b8f 100644
--- a/platform/javascript/id_handler.js
+++ b/platform/javascript/id_handler.js
@@ -28,7 +28,7 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-var IDHandler = function() {
+var IDHandler = /** @constructor */ function() {
 
 	var ids = {};
 	var size = 0;
diff --git a/platform/javascript/os_javascript.cpp b/platform/javascript/os_javascript.cpp
index 9ba02233877..1d7a16db80c 100644
--- a/platform/javascript/os_javascript.cpp
+++ b/platform/javascript/os_javascript.cpp
@@ -935,6 +935,7 @@ Error OS_JavaScript::initialize(const VideoMode &p_desired, int p_video_driver,
 	if (p_desired.fullscreen) {
 		/* clang-format off */
 		EM_ASM({
+			const canvas = Module.canvas;
 			(canvas.requestFullscreen || canvas.msRequestFullscreen ||
 				canvas.mozRequestFullScreen || canvas.mozRequestFullscreen ||
 				canvas.webkitRequestFullscreen
diff --git a/platform/javascript/pre.js b/platform/javascript/pre.js
deleted file mode 100644
index a870e676ea8..00000000000
--- a/platform/javascript/pre.js
+++ /dev/null
@@ -1,5 +0,0 @@
-var Engine = {
-	RuntimeEnvironment: function(Module, exposedLibs) {
-		// The above is concatenated with generated code, and acts as the start of
-		// a wrapper for said code. See engine.js for the other part of the
-		// wrapper.