diff --git a/.circleci/config.yml b/.circleci/config.yml
index ec7b6eb368..294ff225d4 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -30,7 +30,7 @@ jobs:
 
       - run:
           name: Install Emscripten (for GDevelop.js)
-          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 1.39.6 && ./emsdk activate 1.39.6 && cd ..
+          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 3.1.21 && ./emsdk activate 3.1.21 && cd ..
 
       # GDevelop.js dependencies
       - restore_cache:
@@ -107,7 +107,7 @@ jobs:
 
       - run:
           name: Install Emscripten (for GDevelop.js)
-          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 1.39.6 && ./emsdk activate 1.39.6 && cd ..
+          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 3.1.21 && ./emsdk activate 3.1.21 && cd ..
 
       - run:
           name: Install system dependencies for Electron builder
@@ -127,7 +127,8 @@ jobs:
       # Build GDevelop.js (and run tests to ensure it works)
       - run:
           name: Build GDevelop.js
-          command: cd GDevelop.js && source ../emsdk/emsdk_env.sh && npm run build && npm test && cd ..
+          # Use "--runInBand" as it's faster and avoid deadlocks on CircleCI Linux machines (probably because limited in processes number).
+          command: cd GDevelop.js && source ../emsdk/emsdk_env.sh && npm run build && npm test -- --runInBand && cd ..
 
       # GDevelop IDE dependencies (after building GDevelop.js to avoid downloading a pre-built version)
       - run:
@@ -184,7 +185,7 @@ jobs:
 
       - run:
           name: Install Emscripten (for GDevelop.js)
-          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 1.39.6 && ./emsdk activate 1.39.6 && cd ..
+          command: git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 3.1.21 && ./emsdk activate 3.1.21 && cd ..
 
       # GDevelop.js dependencies
       - restore_cache:
@@ -200,7 +201,8 @@ jobs:
       # Build GDevelop.js (and run tests to ensure it works)
       - run:
           name: Build GDevelop.js
-          command: cd GDevelop.js && source ../emsdk/emsdk_env.sh && npm run build && npm test && cd ..
+          # Use "--runInBand" as it's faster and avoid deadlocks on CircleCI Linux machines (probably because limited in processes number).
+          command: cd GDevelop.js && source ../emsdk/emsdk_env.sh && npm run build && npm test -- --runInBand && cd ..
 
       - save_cache:
           paths:
diff --git a/.gitpod.yml b/.gitpod.yml
index f6b7c13f96..348b6b5916 100644
--- a/.gitpod.yml
+++ b/.gitpod.yml
@@ -14,7 +14,7 @@ tasks:
     init: |
           sudo apt-get update 
           sudo apt install cmake python-is-python3 python3-distutils -y
-          git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 1.39.6 && ./emsdk activate 1.39.6 && cd ..
+          git clone https://github.com/juj/emsdk.git && cd emsdk && ./emsdk install 3.1.21 && ./emsdk activate 3.1.21 && cd ..
           cd GDevelop.js
           npm install
           source ../emsdk/emsdk_env.sh && npm run build -- --dev
diff --git a/.travis.yml b/.travis.yml
index eac39ccb16..98ec2f1d36 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,7 +39,7 @@ install:
   - cd ..
 # Install Emscripten (for GDevelop.js)
   - git clone https://github.com/juj/emsdk.git
-  - cd emsdk && ./emsdk install 1.39.6 && ./emsdk activate 1.39.6 && cd ..
+  - cd emsdk && ./emsdk install 3.1.21 && ./emsdk activate 3.1.21 && cd ..
 # Install GDevelop.js dependencies
   - cd GDevelop.js && npm install && cd ..
 # Build GDevelop.js
diff --git a/GDevelop.js/CMakeLists.txt b/GDevelop.js/CMakeLists.txt
index 23f53a7f01..8ab72e1071 100644
--- a/GDevelop.js/CMakeLists.txt
+++ b/GDevelop.js/CMakeLists.txt
@@ -12,8 +12,17 @@ if(NOT EMSCRIPTEN)
 endif()
 
 # Compilation flags (https://emscripten.org/docs/tools_reference/emcc.html):
-add_compile_options(-O2) # Optimizations during compilation
-#add_compile_options(-g --profiling) # Uncomment for debugging + profiling support
+add_compile_options(-O3 -flto) # Optimizations during compilation
+# add_compile_options(-fwasm-exceptions) # Enable exceptions
+if(NOT DISABLE_EMSCRIPTEN_LINK_OPTIMIZATIONS)
+	add_compile_options(-flto) # The compiler needs to know if there will be link time optimisations
+endif()
+
+# Compiler debugging options
+#
+# add_compile_options(-fsanitize=address) # Uncomment to auto-detect occurences of memory bugs (memory leak, use after free, overflows, ...) - also enable linking below!
+# add_compile_options(-fsanitize=undefined) # Uncomment to auto-detect occurences of undefined behavior - also enable linking below!
+# add_compile_options(-g) # Uncomment for debugging support
 # add_compile_options(--profiling) # Uncomment for profiling support
 
 # Common directories:
@@ -64,7 +73,7 @@ if(DISABLE_EMSCRIPTEN_LINK_OPTIMIZATIONS)
 	message(STATUS "Disabling optimization at link time for (slightly) faster build")
 	target_link_libraries(GD "-O0")
 else()
-	target_link_libraries(GD "-O2")
+	target_link_libraries(GD "-O3 -flto")
 endif()
 target_link_libraries(GD "--post-js ${GD_base_dir}/GDevelop.js/Bindings/glue.js")
 target_link_libraries(GD "--post-js ${GD_base_dir}/GDevelop.js/Bindings/postjs.js")
@@ -73,7 +82,17 @@ target_link_libraries(GD "-s EXPORT_NAME=\"initializeGDevelopJs\"") # Global fun
 target_link_libraries(GD "-s TOTAL_MEMORY=48MB") # Get some initial memory size that is a bit bigger than the default.
 target_link_libraries(GD "-s ALLOW_MEMORY_GROWTH=1")
 target_link_libraries(GD "-s ERROR_ON_UNDEFINED_SYMBOLS=0")
-target_link_libraries(GD "-s \"EXTRA_EXPORTED_RUNTIME_METHODS=['addOnPreMain', 'calledRun', 'UTF8ToString']\"")
+target_link_libraries(GD "-s \"EXPORTED_FUNCTIONS=['_free']\"")
+
+# Linker debugging options
+#
+# target_link_libraries(GD "-s DEMANGLE_SUPPORT=1") # Demangle stack traces
+# target_link_libraries(GD "-s ASSERTIONS=1") # Basic runtime memory allocation checks (necessary for wasm exceptions stack traces)
+# target_link_libraries(GD "-s ASSERTIONS=2 -s SAFE_HEAP=1") # Uncomment to do runtime checks for memory allocations and access errors
+# target_link_libraries(GD "-fsanitize=address") # Uncomment to auto-detect occurences of memory bugs (memory leak, use after free, overflows, ...) - also enable compiling above!
+# target_link_libraries(GD "-fsanitize=undefined") # Uncomment to auto-detect occurences of undefined behavior - also enable compiling above!
+# target_link_libraries(--cpuprofiler) # Uncomment for interactive performance profiling
+# target_link_libraries(--memoryprofiler) # Uncomment for interactive memory profiling
 
 # Even if we're building an "executable", prefix it by lib as it's used as a library.
 set_target_properties(GD PROPERTIES PREFIX "lib")
diff --git a/GDevelop.js/Gruntfile.js b/GDevelop.js/Gruntfile.js
index 8d468fad70..a5c6151a43 100644
--- a/GDevelop.js/Gruntfile.js
+++ b/GDevelop.js/Gruntfile.js
@@ -10,7 +10,7 @@ module.exports = function (grunt) {
   const buildOutputPath = '../Binaries/embuild/GDevelop.js/';
   const buildPath = '../Binaries/embuild';
 
-  let cmakeBinary = 'emconfigure cmake';
+  let cmakeBinary = 'emcmake cmake';
   let cmakeGeneratorArgs = [];
   let makeBinary = 'emmake make';
   let makeArgs = ['-j 4'];
@@ -44,11 +44,11 @@ module.exports = function (grunt) {
 
     // Find CMake in usual folders or fallback to PATH.
     if (fs.existsSync('C:\\Program Files\\CMake\\bin\\cmake.exe')) {
-      cmakeBinary = 'emconfigure "C:\\Program Files\\CMake\\bin\\cmake"';
+      cmakeBinary = 'emcmake "C:\\Program Files\\CMake\\bin\\cmake"';
     } else if (
       fs.existsSync('C:\\Program Files (x86)\\CMake\\bin\\cmake.exe')
     ) {
-      cmakeBinary = 'emconfigure "C:\\Program Files (x86)\\CMake\\bin\\cmake"';
+      cmakeBinary = 'emcmake "C:\\Program Files (x86)\\CMake\\bin\\cmake"';
     } else {
       console.log(
         "⚠️ Can't find CMake in its usual Program Files folder. Make sure you have cmake in your PATH instead."
diff --git a/GDevelop.js/README.md b/GDevelop.js/README.md
index 3bf45ced4c..f884a11b32 100644
--- a/GDevelop.js/README.md
+++ b/GDevelop.js/README.md
@@ -9,24 +9,30 @@ This is the port of GDevelop core classes to WebAssembly+JavaScript. This allows
 > 👋 Usually, if you're working on the GDevelop editor or extensions in JavaScript, you don't need to rebuild GDevelop.js. If you want to make changes in C++ extensions or classes, read this section.
 
 - Prerequisite tools installed:
+
   - [CMake 3.17+](http://www.cmake.org/) (3.5+ should work on Linux/macOS). On macOS, you can install it via Homebrew (recommended for Apple M1 Architectures).
   - [Node.js](https://nodejs.org/). (We recommend using [nvm](https://github.com/nvm-sh/nvm) to be able to switch between Node versions easily).
   - Python (via [pyenv](https://github.com/pyenv/pyenv) for versions management).
 
-- Install [Emscripten](https://github.com/kripken/emscripten), as explained below or on the [Emscripten installation instructions](http://kripken.github.io/emscripten-site/docs/getting_started/downloads.html):
+- Install [Emscripten](https://github.com/kripken/emscripten) version `3.1.21`, as explained below or on the [Emscripten installation instructions](http://kripken.github.io/emscripten-site/docs/getting_started/downloads.html):
 
-| Linux/macOS                                  | Windows                                      |
-| -------------------------------------------- | -------------------------------------------- |
-| `git clone https://github.com/juj/emsdk.git` | `git clone https://github.com/juj/emsdk.git` |
-| `cd emsdk`                                   | `cd emsdk`                                   |
-| `git pull`                                   | `git pull`                                   |
-| `./emsdk install 1.39.6`                     | `emsdk install 1.39.6`                       |
-| `./emsdk activate 1.39.6`                    | `emsdk activate 1.39.6`                      |
-| `source ./emsdk_env.sh`                      | `emsdk_env.bat`                              |
+```bash
+git clone https://github.com/emscripten-core/emsdk/
+cd emsdk
+git pull
+./emsdk install 3.1.21
+./emsdk activate 3.1.21
+```
 
 > ⚠️ If you are on Apple M1, this version of emsdk may not be available for this architecture and you will get an error when installing it, indicating a missing binary. If this is the case, a workaround is to modify the `emsdk.py` line 132 with `machine = 'x86_64'` to ensure a correct binary is downloaded.
 
-- Launch the build from GDevelop.js folder:
+- Whenever you try to build GDevelop.js in the future, you will have to load the emsdk environement into your terminal window again by running:
+
+| Linux/macOS             | Windows (Powershell) | Windows (cmd.exe) |
+| ----------------------- | -------------------- | ----------------- |
+| `source ./emsdk_env.sh` | `./emsdk_env.ps1`    | `./emsdk_env.bat` |
+
+- With the emscripten environement loaded into your terminal, launch the build from GDevelop.js folder:
 
 ```bash
 cd GDevelop.js
@@ -38,6 +44,8 @@ npm run build
 
 > ℹ️ Output is created in _/path/to/GD/Binaries/embuild/GDevelop.js/_ and also copied to GDevelop 5 IDE (`newIDE` folder).
 
+> ℹ️ You only need to run `npm install` the first time you make a build.
+
 -> ⏱ The linking (last step) of the build can be made a few seconds faster by specifying `-- --dev`. Be sure to remove it before building a release version, as this disable "link-time optimizations" of the generated WebAssembly module.
 
 - You can then launch GDevelop 5 that will use your build of GDevelop.js:
diff --git a/GDevelop.js/ninja/ninja.exe b/GDevelop.js/ninja/ninja.exe
index 3c95c0a3c7..e4fafa04c3 100644
Binary files a/GDevelop.js/ninja/ninja.exe and b/GDevelop.js/ninja/ninja.exe differ
diff --git a/appveyor.yml b/appveyor.yml
index 77ebe04e48..53100cfd97 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -19,8 +19,6 @@ init:
 install:
   # Build GDevelop.js (and run tests to ensure it works).
   # (in a subshell to avoid Emscripten polluting the Node.js and npm version for the rest of the build)
-  # Also install setuptools as our old version of Emscripten uses distutils which was removed in Python 3.12.
-  # setuptools will make distutils available again (but we should migrate to a newer Emscripten version).
   - cmd: >-
       cd GDevelop.js
 
@@ -30,16 +28,18 @@ install:
 
       cd emsdk
 
-      emsdk install 1.39.6
-
-      CMD /C "emsdk activate 1.39.6 && pip install setuptools && cd .. && npm run build"
-
+      emsdk install 3.1.21
 
+      CMD /C "emsdk activate 3.1.21 && cd .. && npm run build"
 
       cd ..\..
 
-  # Build GDevelop IDE
+  # Build GDevelop IDE.
+  # Also install setuptools as something requires distutils in electron-app, and it was removed in Python 3.12.
+  # setuptools will make distutils available again (but we should migrate our packages probably).
   - cmd: >-
+      pip install setuptools
+  
       cd newIDE\app
 
       npm -v && npm install