declare const enum values not inlined - ReferenceError at runtime

[jrieken]

Summary

esbuild does not inline declare const enum member values at usage sites. TypeScript does — it replaces usages like Foo.A with the literal value 1. esbuild leaves the enum identifier as-is, causing a ReferenceError at runtime since declare means no enum object is emitted.

Repro

// input.ts
export declare const enum Foo { A = 1, B = 2 }

export function test() {
  console.log(Foo.A);
}

Expected output (what tsc produces):

export function test() {
  console.log(1 /* Foo.A */);
}

Actual esbuild output:

export function test() {
  console.log(Foo.A); // ReferenceError: Foo is not defined
}

Note: esbuild correctly inlines const enum (without declare). The bug is specific to the declare modifier. Both const enum and declare const enum should have the same runtime semantics — values inlined, no enum object emitted.

esbuild playground link

https://esbuild.github.io/try/#YgAwLjI3LjAALS10YXJnZXQ6ZXMyMDI0AGUAZW50cnkudHMAZXhwb3J0IGRlY2xhcmUgY29uc3QgZW51bSBGb28geyBBID0gMSwgQiA9IDIgfQoKZXhwb3J0IGZ1bmN0aW9uIHRlc3QoKSB7CiAgY29uc29sZS5sb2coRm9vLkEpOwp9

Impact

This causes runtime crashes in VS Code (microsoft/vscode#295046). We are working around it by removing declare from our const enum declarations.

Environment

• esbuild 0.27.0
• Using esbuild.transform() with loader: 'ts', target: 'es2024'
• Also reproducible with esbuild.build()

[evanw]

Using declare on a declaration puts it into the type namespace instead of the value namespace. Features that use the TypeScript type system are deliberately not supported by esbuild because replicating TypeScript's type system into esbuild is out of scope. This is not a bug with esbuild.

I recommend removing the declare from your const enum decorations to avoid unsupported syntax constructs.

[jrieken]

declare from your const enum decorations to avoid unsupported syntax constructs.

Yeah, that's what I did and I am also convinced we didn't add the declare for any good reason. Just want to let you know with this issue and I don't mind it getting closed without action.

btw - all of bundling and transpiling VS Code Core is now based on esbuild. So, many thanks and much love for this awesome tool.