For host environments like the web browser and nodejs, build as a library using the freestanding OS target. Here’s an example of running Zig code compiled to WebAssembly with nodejs.

test.js

const fs = require('fs');
const source = fs.readFileSync("./math.wasm");
const typedArray = new Uint8Array(source);
WebAssembly.instantiate(typedArray, {
  env: {
    print: (result) => { console.log(`The result is ${result}`); }
  }}).then(result => {
  const add = result.instance.exports.add;
  add(1, 2);
});

WASI

args.zig

    var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
    const gpa = &general_purpose_allocator.allocator;
    const args = try std.process.argsAlloc(gpa);
    defer std.process.argsFree(gpa, args);
    for (args) |arg, i| {
        std.debug.print("{}: {}\n", .{ i, arg });
    }
}

$ zig build-exe args.zig -target wasm32-wasi

preopens.zig

const std = @import("std");
const PreopenList = std.fs.wasi.PreopenList;
pub fn main() !void {
    var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
    const gpa = &general_purpose_allocator.allocator;
    var preopens = PreopenList.init(gpa);
    defer preopens.deinit();
    try preopens.populate();
    for (preopens.asSlice()) |preopen, i| {
        std.debug.print("{}: {}\n", .{ i, preopen });
    }
}