Duplicate Identifier in generated output

[gliheng]

Bug report

Version (5.2.1)

terser --module --compress --mangle -- ./input.js > output.js

terser input

let __VITE_CSS__ = document.createElement("style");
__VITE_CSS__.innerHTML = "";
document.head.appendChild(__VITE_CSS__);
let wasm;
let cachedTextDecoder = new TextDecoder("utf-8", {ignoreBOM: true, fatal: true});
cachedTextDecoder.decode();
let cachegetUint8Memory0 = null;
function getUint8Memory0() {
  if (cachegetUint8Memory0 === null || cachegetUint8Memory0.buffer !== wasm.memory.buffer) {
    cachegetUint8Memory0 = new Uint8Array(wasm.memory.buffer);
  }
  return cachegetUint8Memory0;
}
function getStringFromWasm0(ptr, len) {
  return cachedTextDecoder.decode(getUint8Memory0().subarray(ptr, ptr + len));
}
const heap = new Array(32).fill(void 0);
heap.push(void 0, null, true, false);
let heap_next = heap.length;
function addHeapObject(obj) {
  if (heap_next === heap.length)
    heap.push(heap.length + 1);
  const idx = heap_next;
  heap_next = heap[idx];
  heap[idx] = obj;
  return idx;
}
function getObject(idx) {
  return heap[idx];
}
function dropObject(idx) {
  if (idx < 36)
    return;
  heap[idx] = heap_next;
  heap_next = idx;
}
function takeObject(idx) {
  const ret = getObject(idx);
  dropObject(idx);
  return ret;
}
function greet() {
  wasm.greet();
}
function main_js() {
  wasm.main_js();
}
async function load(module, imports) {
  if (typeof Response === "function" && module instanceof Response) {
    if (typeof WebAssembly.instantiateStreaming === "function") {
      try {
        return await WebAssembly.instantiateStreaming(module, imports);
      } catch (e) {
        if (module.headers.get("Content-Type") != "application/wasm") {
          console.warn("`WebAssembly.instantiateStreaming` failed because your server does not serve wasm with `application/wasm` MIME type. Falling back to `WebAssembly.instantiate` which is slower. Original error:\n", e);
        } else {
          throw e;
        }
      }
    }
    const bytes = await module.arrayBuffer();
    return await WebAssembly.instantiate(bytes, imports);
  } else {
    const instance = await WebAssembly.instantiate(module, imports);
    if (instance instanceof WebAssembly.Instance) {
      return {instance, module};
    } else {
      return instance;
    }
  }
}
async function init(input) {
  if (typeof input === "undefined") {
    input = "/_assets/rust_crate_bg.0ccbb076.wasm";
  }
  const imports = {};
  imports.wbg = {};
  imports.wbg.__wbg_alert_704a1ef95c5aac35 = function(arg0, arg1) {
    alert(getStringFromWasm0(arg0, arg1));
  };
  imports.wbg.__wbindgen_string_new = function(arg0, arg1) {
    var ret = getStringFromWasm0(arg0, arg1);
    return addHeapObject(ret);
  };
  imports.wbg.__wbg_log_61ea781bd002cc41 = function(arg0) {
    console.log(getObject(arg0));
  };
  imports.wbg.__wbindgen_object_drop_ref = function(arg0) {
    takeObject(arg0);
  };
  imports.wbg.__wbindgen_throw = function(arg0, arg1) {
    throw new Error(getStringFromWasm0(arg0, arg1));
  };
  if (typeof input === "string" || typeof Request === "function" && input instanceof Request || typeof URL === "function" && input instanceof URL) {
    input = fetch(input);
  }
  const {instance, module} = await load(await input, imports);
  wasm = instance.exports;
  init.__wbindgen_wasm_module = module;
  wasm.__wbindgen_start();
  return wasm;
}
export default init;
export {greet, main_js};

terser output or error

let e,n=document.createElement("style");n.innerHTML="",document.head.appendChild(n);let t=new TextDecoder("utf-8",{ignoreBOM:!0,fatal:!0});t.decode();let a=null;function i(n,i){return t.decode((null!==a&&a.buffer===e.memory.buffer||(a=new Uint8Array(e.memory.buffer)),a).subarray(n,n+i))}const o=new Array(32).fill(void 0);o.push(void 0,null,!0,!1);let r=o.length;function s(e){return o[e]}function c(e){const n=s(e);return function(e){e<36||(o[e]=r,r=e)}(e),n}function u(){e.greet()}function f(){e.main_js()}export default async function n(t){void 0===t&&(t="/_assets/rust_crate_bg.0ccbb076.wasm");const a={wbg:{}};a.wbg.__wbg_alert_704a1ef95c5aac35=function(e,n){alert(i(e,n))},a.wbg.__wbindgen_string_new=function(e,n){return function(e){r===o.length&&o.push(o.length+1);const n=r;return r=o[n],o[n]=e,n}(i(e,n))},a.wbg.__wbg_log_61ea781bd002cc41=function(e){console.log(s(e))},a.wbg.__wbindgen_object_drop_ref=function(e){c(e)},a.wbg.__wbindgen_throw=function(e,n){throw new Error(i(e,n))},("string"==typeof t||"function"==typeof Request&&t instanceof Request||"function"==typeof URL&&t instanceof URL)&&(t=fetch(t));const{instance:u,module:f}=await async function(e,n){if("function"==typeof Response&&e instanceof Response){if("function"==typeof WebAssembly.instantiateStreaming)try{return await WebAssembly.instantiateStreaming(e,n)}catch(n){if("application/wasm"==e.headers.get("Content-Type"))throw n;console.warn("`WebAssembly.instantiateStreaming` failed because your server does not serve wasm with `application/wasm` MIME type. Falling back to `WebAssembly.instantiate` which is slower. Original error:\n",n)}const t=await e.arrayBuffer();return await WebAssembly.instantiate(t,n)}{const t=await WebAssembly.instantiate(e,n);return t instanceof WebAssembly.Instance?{instance:t,module:e}:t}}(await t,a);return e=u.exports,n.__wbindgen_wasm_module=f,e.__wbindgen_start(),e}export{u as greet,f as main_js};

Expected result
In the output js, there is a let n and a function n. When loaded in browser as a module, it lead to this error: output.js:formatted:1 Uncaught SyntaxError: Identifier 'n' has already been declared.

[fabiosantoscode]

This bug surprised me!

The reason for this is that Terser turns function declarations in export default into function expressions. But only when they are moved there

I got the repro down to:

let e = eeeeepleeeease();
eeepleeeease(e)
function also_e() {
  return also_e;
}
export default also_e;

[fabiosantoscode]

A fix has been pushed, which will be released tomorrow.

[developit]

I think there's another cases where this happens, but I can't seem to narrow it down. Running Terser (4.8 or 5.3) on this file causes the first imported identifier to become e, but also uses that identifier for the default-exported function's name:

https://unpkg.com/@emotion/styled-base@10.0.31/dist/styled-base.esm.js

import e from"/@npm/@babel/runtime/helpers/defineProperty";
// ...
export default function e(t,p){ // ...

[fabiosantoscode]

Reopening for investigation, thanks @developit

[fabiosantoscode]

@developit it's a variation on the previous issue :) an export default function x() {} counts as a function declaration.

Minimal repro:

import ee from 'ee';

var also_e = function also_e() {
  return also_e;
};

export default also_e;

Once the function expression with the name also_e gets inlined into export default, it becomes a function declaration, but Terser doesn't know that right now.