This is unpublished documentation of working with Rust and WebAssembly, the published documentation is available on the main Rust and WebAssembly documentation site . Features documented here may not be available in released versions of tooling for Rust and WebAssembly.


Before we write much more code, we will want to have some debugging tools in our belt for when things go wrong. Take a moment to review the reference page listing tools and approaches available for debugging Rust-generated WebAssembly.

Enable Logging for Panics

If our code panics, we want informative error messages to appear in the developer console.

Our wasm-pack-template comes with an optional, enabled-by-default dependency on the console_error_panic_hook crate that is configured in wasm-game-of-life/src/ All we need to do is install the hook in an initialization function or common code path. We can call it inside the Universe::new constructor in wasm-game-of-life/src/

# #![allow(unused_variables)]
#fn main() {
pub fn new() -> Universe {

    // ...

Add Logging to our Game of Life

Let's use the console.log function via the web-sys crate to add some logging about each cell in our Universe::tick function.

First, add web-sys as a dependency and enable its "console" feature in wasm-game-of-life/Cargo.toml:

version = "0.3"
features = [

For ergonomics, we'll wrap the console.log function up in a println!-style macro:

# #![allow(unused_variables)]
#fn main() {
extern crate web_sys;

// A macro to provide `println!(..)`-style syntax for `console.log` logging.
macro_rules! log {
    ( $( $t:tt )* ) => {
        web_sys::console::log_1(&format!( $( $t )* ).into());

Now, we can start logging messages to the console by inserting calls to log in Rust code. For example, to log each cell's state, live neighbors count, and next state, we could modify wasm-game-of-life/src/ like this:

diff --git a/src/ b/src/
index f757641..a30e107 100755
--- a/src/
+++ b/src/
@@ -123,6 +122,14 @@ impl Universe {
                 let cell = self.cells[idx];
                 let live_neighbors = self.live_neighbor_count(row, col);

+                log!(
+                    "cell[{}, {}] is initially {:?} and has {} live neighbors",
+                    row,
+                    col,
+                    cell,
+                    live_neighbors
+                );
                 let next_cell = match (cell, live_neighbors) {
                     // Rule 1: Any live cell with fewer than two live neighbours
                     // dies, as if caused by underpopulation.
@@ -140,6 +147,8 @@ impl Universe {
                     (otherwise, _) => otherwise,

+                log!("    it becomes {:?}", next_cell);
                 next[idx] = next_cell;

Using a Debugger to Pause Between Each Tick

Browser's stepping debuggers are useful for inspecting the JavaScript that our Rust-generated WebAssembly interacts with.

For example, we can use the debugger to pause on each iteration of our renderLoop function by placing a JavaScript debugger; statement above our call to universe.tick().

const renderLoop = () => {



This provides us with a convenient checkpoint for inspecting logged messages, and comparing the currently rendered frame to the previous one.

Screenshot of debugging the Game of Life


  • Add logging to the tick function that records the row and column of each cell that transitioned states from live to dead or vice versa.

  • Introduce a panic!() in the Universe::new method. Inspect the panic's backtrace in your Web browser's JavaScript debugger. Disable debug symbols, rebuild without the console_error_panic_hook optional dependency, and inspect the stack trace again. Not as useful is it?