Svelte-5, top level await & ESM output

[k-dahl]

I'm unable to deploy svelte-5 with this adapter and the new svelte.js/ts files with top-level await because the lambda here gets compiled as CJS, which does not support top level await.

I tried passing in esbuild: { format: 'esm' } via the adapter config, and that gets me a little further with a successful deploy - but the resulting lambda function then fails on every request with:

{
    "errorType": "Runtime.UserCodeSyntaxError",
    "errorMessage": "SyntaxError: Cannot use import statement outside a module",
    "stack": [
        "Runtime.UserCodeSyntaxError: SyntaxError: Cannot use import statement outside a module",
        "    at _loadUserApp (file:///var/runtime/index.mjs:1084:17)",
        "    at async UserFunction.js.module.exports.load (file:///var/runtime/index.mjs:1119:21)",
        "    at async start (file:///var/runtime/index.mjs:1282:23)",
        "    at async file:///var/runtime/index.mjs:1288:1"
    ]
}

my package.json does have "type": "module" but I suspect the issue is specific to the esbuild options.

Any insights you could provide would be greatly appreciated!

[k-dahl]

I was able to get this a bit farther by manually adding a package.json alongside the server.js output, it only has { "type": "module" } in it... that then results in a new error about not being able to require "path", which I as able to resolve with this shim via banner/js:

            esbuild: {
                format: 'esm',
                platform: 'node',
                target: 'esnext',
                minify: false,
                banner: {
                    js: "import { createRequire } from 'module';const require = createRequire(import.meta.url);",
                }
            },

so, it is working now but still requires the manual step of creating that package.json file after each deploy.

[k-dahl]

I switched the adapter to deploy: false, and added some additional build steps to stick a simple package.json file into the lambda output directory, and then run the CDK deploy in a separate build step. It does seem to be working, but I'd love to hear if there is any other way to do this!