fix(build): declare preload-helper has no side effects

[yuzheng14]

Description

When there's is at least one dynamic import and the modulePreload is false, it will introduce a detectScriptRel function. But when the content imported dynamically is unused, the detectScriptRel will not be tree-shaken.

Minimum reproductible set: https://stackblitz.com/edit/vitejs-vite-8iwu7r?file=src%2Fmain.ts

There is a b.ts which exports a b:

export const b = 2;

And a a.ts which exports a a and a b that dynamically imports b from b.ts:

export const a = 1;

export async function b() {
  const b = (await import('./b.ts')).b;
  return b;
}

Then only use a in entry main.ts.

import { a } from './a.ts';

console.log(a);

Finally bundle it. It will present a IIFE of detectScriptRel in the product:

(function detectScriptRel() {
  const relList = typeof document !== "undefined" && document.createElement("link").relList;
  return relList && relList.supports && relList.supports("modulepreload") ? "modulepreload" : "preload";
})();
const a = 1;
console.log(a);

And the config of vite is:

import { defineConfig } from 'vite';

export default defineConfig({
  build: {
    minify: false,
    modulePreload: false,
  },
});

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[sapphi-red]

Thanks!