Debugging Story: Build failed, error from Terser

January 8, 2020

The following is a record of the steps I went through when debugging a build-time bug I encountered during work.

It all started with an error message during the build.

ERROR in bundle.xxxx.js from Terser


Command failed with exit code 2
Visit for documentation about this command.
Error: Command failed: yarn build

What is wrong with Terser and our code?

I must find the root cause of this!

Act I - The First Attempt

We used Terser to minify our build code. It was part of our webpack pipeline, installed through terser-webpack-plugin. Since terser is throwing an error, so I disabled terser-webpack-plugin and build again.
module.exports = {
  plugins: [
    // ...
    // disable terser first
    // new TerserPlugin({
    // 	terserOptions: {
    //     // ...
    //   },
    // }),

Ok. The build was successful. Nothing wrong with the code nor the build.

So, I run the terser manually:

const terser = require('terser');
const fs = require('fs');
const path = require('path');
const input = fs.readFileSync(
  path.join(__dirname, 'dist/bundle.xxxx.js'),

const output = terser.minify(input, {
  // ...terser options

The console output an error that was cryptic

  TypeError: Cannot read property 'name' of undefined
      at A.f (node_modules/terser/dist/bundle.js:44:146028)
      at D (node_modules/terser/dist/bundle.js:44:2863)
      at A (node_modules/terser/dist/bundle.js:44:146075)
      at k (node_modules/terser/dist/bundle.js:44:146998)
      at vn (node_modules/terser/dist/bundle.js:44:133003)
      at node_modules/terser/dist/bundle.js:44:165250
      at AST_BlockStatement.optimize (node_modules/terser/dist/bundle.js:44:121400)
      at Ct.before (node_modules/terser/dist/bundle.js:44:121157)
      at AST_BlockStatement.transform (node_modules/terser/dist/bundle.js:44:78200)
      at node_modules/terser/dist/bundle.js:44:78849

I clicked into the terser source code to see what was going wrong over there. (Maybe it's my chance to contribute to terser?)

Little did I know, how wrong I could be.

terser/dist/bundle.js was a minified code and it wasn't meant for humans eyes 😭

Allow me to share a small snippet of what I saw:

s) } else if (u instanceof Se && r === u.expression && (p(e, t, i, u, o = n(o,, a + 1,
  s + 1), o)) return; a > 0 || u instanceof Be && r !== u.tail_node() || u instanceof S ||
(t.direct_access = !0) } e(F, u); var d = new qn(function (e) { if (e instanceof Je) { var n =
e.definition(); n && (e instanceof hn && n.references.push(e), n.fixed = !1) } }); function h(e, n,
t) { this.inlined = !1; var r = e.safe_ids; return e.safe_ids = Object.create(null), i(e, t, this),
n(), e.safe_ids = r, !0 } function m(e, n, t) { var r, o = this; return o.inlined = !1, a(e), i(e,
t, o), ! && (r = e.parent()) instanceof ke && r.expression === o && o.argnames.forEach(
function (n, t) { if (n.definition) { var i = n.definition(); void 0 !== i.fixed || o.uses_arguments
&& !e.has_directive("use strict") ? i.fixed = !1 : (i.fixed = function () { return r.args[t] || v(kn, r)

At this point in time, I had another meeting to attend, so I stopped at this juncture.

On my way there, I was thinking, well, maybe I should probe this later in another direction.

Act II - terser-webpack-plugin

After the meeting, I was thinking, well let me reenable the terser plugin, and try to probe through the plugin.

Remember the error message: "ERROR in bundle.xxxx.js from Terser" ?

I looked into the node_modules/terser-webpack/plugin/dist/index.js and search for the word "from Terser".

Tip: Usually when you want to debug a library installed in node_modules, you can first look at the package.json, it usually has an entry called "main" that tells you the entry file to first look into. The main file usually exports all the public API that you use, so it is a good place to start diving into.

So I found the line:

return new Error(`${file} from Terser\n${err.message}`);

After adding logs console.log(err) and run the build again, I realised the err is an empty object {}, which explained why I saw undefined after the ERROR in ... from Terser.

So, I slowly traced back the caller, and find out where this err object was initially created.

Tip: To trace the callers of a function that leads up to a certain state of your application, you can:

function someFunction() {
  if (someConditionThatLeadsToErrorLaterOn) {
    try {
      throw new Error();
    } catch (error) {

This is especially useful if you are tracing an unfamiliar code, you can quickly get a call stack that leads up to the current condition.

After tracing through the call stack, I ended up at the line where the terser-webpack-plugin calls the terser, and when I logged out the error, it shows:

  TypeError: Cannot read property 'name' of undefined
      at A.f (node_modules/terser/dist/bundle.js:44:146028)
      at D (node_modules/terser/dist/bundle.js:44:2863)
      at A (node_modules/terser/dist/bundle.js:44:146075)
      at k (node_modules/terser/dist/bundle.js:44:146998)
      at vn (node_modules/terser/dist/bundle.js:44:133003)
      at node_modules/terser/dist/bundle.js:44:165250
      at AST_BlockStatement.optimize (node_modules/terser/dist/bundle.js:44:121400)
      at Ct.before (node_modules/terser/dist/bundle.js:44:121157)
      at AST_BlockStatement.transform (node_modules/terser/dist/bundle.js:44:78200)
      at node_modules/terser/dist/bundle.js:44:78849

So familiar! After an hour of tracing and debugging, I ended up at the same place.

Note: the error must have lost somewhere from the terser to the actual print out of terser-webpack-plugin, it might have fixed in a later version of terser-webpack-plugin, but I'm not sure of it yet. Anyone interested can help check.

Act III - Terser

The circumstances left me with no choice. I needed to face the cryptic minified code.

Luckily VSCode still able to open the huge minified file, and able to set the cursor to the right line and column:

var f = n.option("ecma") < 6 && n.has_directive("use strict") ?
function (e) { return e.key!=u &&!=u }

(By the way, in the minified code, all the code is in one line. 🤦‍♂️)

Well, this may seem like the right place to throw the "Cannot read property 'name' of undefined" error.

To understand what is going on in this line, I cloned terser, checked out to the version tag that was installed in our codebase, and tried to figure out where that line was in the original code.

Tip: String, property and method names are usually the best marker to trace a mangled code. Even though all the variables have mangled into a single character variable name, you can still clearly see the method has_directive() and the string "use strict".

Conversely, please don't write long windy property / method names, it doesn't mangle well.

So I global searched the keyword has_directive("use strict") and landed with a small number of results, which I looked through every one of them and ended up with the following line:

var diff =
  compressor.option('ecma') < 6 && compressor.has_directive('use strict')
    ? function(node) {
        return node.key != prop && != prop;
    : function(node) {
        return != prop;

Which I was and still am clueless of what this code was trying to do.

So I did the most reasonable thing, add a console.log(node).

var f = n.option("ecma") < 6 && n.has_directive("use strict") ?
function (e) { if (!e.key) { console.log(e); } return e.key!=u
&&!=u }

I found out that the node is an object, that does not have a property key, which explains the error. And node has a property call name that has a value "foobar", which I assumed is a variable name in our codebase. Luckily foobar wasn't commonly used in our codebase, and I managed to find only 1 instance of it, and astonishingly, the code was last changed 1 year ago!

So Terser just decided to break, without a sign, on a line of code that was written 1 year ago. This is the life of a programmer.

Final Act - The Resolution

I kind of concluded that the root cause was a Terser bug, (because I can't just change the code that wasn't touched for nearly 1 year for no good reason), so the obvious thing to do next was to figured out whether someone fixed it on Terser upstream.

So, I checked out the master branch of Terser, found out the code has changed to

var diff = compressor.option("ecma") < 2015
    && compressor.has_directive("use strict") ? function(node) {
    return node.key != prop && (node.key && != prop);
} : function(node) {
    return node.key && != prop;

node.key is checked to be existed before checking What a simple patch!

The next thing I needed to figure out was when was this fix landed, whether I can upgrade it.

The Terser in the codebase was one major version behind the latest Terser version, so, I was more reserved to upgrade to the latest version.

The git blame for the line of code was for some code refactoring, so I went to Github to trace the blame.

Tip: Github blame has this very useful button, that allows you to view blame prior to the change.

Github: view blame prior to the change

A few blame traces later, I ended up with a commit that fixed the bug:

fix node.key crashing lib/compress by hytromo · Pull Request #286 · terser/terser

By looking at the MR merged date, I found that the commit was landed in between v3.16.0 and v3.17.0.

v3.17.0 was a minor version bump for our codebase, so I assumed it has no breaking changes.

terser was installed as a dependency of terser-webpack-plugin, which we had no control on the terser version, so I added a resolution to our package.json:

  "resolution": {
    "terser": "3.17.0"

After I upgraded terser, I build the code again.

The build was successful! 🎉

Closing Note

As I was explaining all these to my colleague, I realised that should I upgraded the terser once I found out that it was a terser error, it would have fixed the bug as well. I wouldn't need to go through all these to end up in the same fix.

Oh well. 🤷‍♂️