For the workaround, see #399 (comment).

Consider the following code:

// entry1.js
import "./init-dep-1.js";
import "./run-dep.js";

// entry2.js
import "./init-dep-2.js";
import "./run-dep.js";

// init-dep-1.js
global.foo = {
  log: () => console.log("foo.log() (from entry 1) called"),
};

// init-dep-2.js
global.foo = {
  log: () => console.log("foo.log() (from entry 2) called"),
};

// run-dep.js
global.foo.log();

When you bundle this code with esbuild --bundle --outdir=build --format=esm --splitting --platform=node --out-extension:.js=.mjs ./entry1.js ./entry2.js, ESBuild discovers that run-dep.js is a module shared between both entry points. Because code splitting is enabled, ESBuild moves it into a separate chunk:

// build/entry1.js
import "./chunk.P2RPFHF3.js";

global.foo = {
  log: () => console.log("foo.log() (from entry 1) called")
};

However, by doing so, ESBuild puts run-dep.js above the init-dep-1.js or init-dep-2.js code. This changes the import order – and breaks the code:

$ esbuild --bundle --outdir=build --format=esm --splitting --platform=node --out-extension:.js=.mjs ./entry1.js ./entry2.js
$ node build/entry1.mjs
global.foo.log();
           ^

TypeError: Cannot read property 'log' of undefined