Add a shim function for unbundled uses of require

[evanw]

Modules in CommonJS format automatically get three variables injected into their scope: module, exports, and require. These allow the code to import other modules and to export things from itself. The bundler automatically rewrites uses of module and exports to refer to the module's exports and certain uses of require to a helper function that loads the imported module.

Not all uses of require can be converted though, and un-converted uses of require will end up in the output. This is problematic because require is only present at run-time if the output is run as a CommonJS module. Otherwise require is undefined, which means esbuild's behavior is inconsistent between compile-time and run-time. The module and exports variables are objects at compile-time and run-time but require is a function at compile-time and undefined at run-time. This causes code that checks for typeof require to have inconsistent behavior:

if (typeof require === 'function' && typeof exports === 'object' && typeof module === 'object') {
  console.log('CommonJS detected')
}

In the above example, ideally CommonJS detected would always be printed since the code is being bundled with a CommonJS-aware bundler. To fix this, esbuild will now substitute references to require with a stub __require function when bundling if the output format is something other than CommonJS. This should ensure that require is now consistent between compile-time and run-time. When bundled, code that uses unbundled references to require will now look something like this:

var __require = (x) => {
  if (typeof require !== "undefined")
    return require(x);
  throw new Error('Dynamic require of "' + x + '" is not supported');
};
var __commonJS = (cb, mod) => () => (mod || cb((mod = {exports: {}}).exports, mod), mod.exports);
var require_example = __commonJS((exports, module) => {
  if (typeof __require === "function" && typeof exports === "object" && typeof module === "object") {
    console.log("CommonJS detected");
  }
});
require_example();

Fixes #1202

[arcanis]

Annoyingly, this breaks passing the output of ESBuild into Webpack (like we do in our incremental migration, until all the loaders we need are available), since Webpack reports the shim as a fully dynamic require and is unable to apply static analysis to the require we marked as external.

[anchengjian]

how to remove this shim function ???

[tuchuants]

This commit broke require.resolve functionality.