Emitting an Executable

For a single file you do not need a build script at all:

zig build-exe main.zig          # an executable
zig build-lib  main.zig         # a static library
zig build-obj  main.zig         # an object file
zig run        main.zig         # build and run in one step
zig test       main.zig         # build and run its tests

Useful flags

Targeting WebAssembly

This is how every snippet on this site is built:

zig build-exe main.zig -target wasm32-wasi -OReleaseSmall

For a freestanding wasm module with no WASI (the browser-canvas case), you want -target wasm32-freestanding plus -fno-entry and explicit exports:

zig build-exe main.zig -target wasm32-freestanding \
  -OReleaseSmall -fno-entry --export=add

When to graduate to build.zig

Once you have more than one artifact, dependencies, generated files, or a test step worth naming, move to a build script. zig build --help then lists the steps and options that script defines.