It seems like esbuild generally inlines IIFEs, but at least sometimes this does not seem to be happening. One example:

// Input

function x(): string {
  console.log('x');
  return 'x';
}

function y(): void {
  const cb = () => {
    console.log(x());
  };

  return cb();
}

y();

// Current output

function x() {
  return console.log("x"), "x";
}
function y() {
  // Unnecessary `return` and IIFE
  return (() => {
    console.log(x());
  })();
}
y();

// Expected output

function x() {
  return console.log("x"), "x";
}
function y() {
  console.log(x());
}
y();

Playground link

Ideally, this should be able to further inline the only calls of x and y into:

// Ideal output

console.log(console.log("x"), "x");

This is may be related in terms of function call inline, but is potentially more involved than just inlining an IIFE.

I ran into this today when attempting to conditionally wrap a function based on a --define value like so:

declare global {
  var isDebug: boolean | undefined;
}

function doSomething(): void {
  const callback = () => {
    console.log('something');
  };

  if (isDebug) {
    callWithDebugInfo(callback);
  } else {
    callback();
  }
}

function callWithDebugInfo(callback: () => void): void {
  setDebugInfo({/* ... */});
  try {
    callback();
  } finally {
    removeDebugInfo({/* ... */});
  }
}

esbuild is able to correctly tree-shake callWithDebugInfo with --define:isDebug=false, but this leaves an extra IIFE in the production optimization (unable to properly inline callback()), which is a cost I'd prefer not to pay for this debug feature.