Function names are not properly minified and causes a invalid syntax when ecma=5 is provided

[JayaKrishnaNamburu]

Bug

Version ^5.7.0

{ keep_fnames: true, keep_classnames: true } 

These values are initially passed inside the terser-webpack-plugin. https://github.com/JayaKrishnaNamburu/terser-minifier-bug-reproduction/blob/main/webpack/webpack.config.prod.js#L18

I am using terser alongside webpack via terser-webpack-plugin and when built for production. The function is replaced with some for loop with a invalid syntax

terser input
Terser input was from a npm module. And the raw file can be found here https://unpkg.com/browse/@teleporthq/teleport-plugin-common@0.17.0/dist/esm/index.js

// packages/teleport-plugin-common/src/utils/hast-utils.ts
var hast_utils_exports = {};
teleport_plugin_common_dist_esm_export(hast_utils_exports, {
  addAttributeToNode: () => addAttributeToNode,
  addBooleanAttributeToNode: () => addBooleanAttributeToNode,
  addChildNode: () => addChildNode,
  addClassToNode: () => addClassToNode,
  addTextNode: () => addTextNode
});
var addBooleanAttributeToNode = (node, key, value = true) => {
  node.properties[key] = value === true ? "" : false;
};
var addAttributeToNode = (node, key, value) => {
  node.properties[key] = value;
};
var addClassToNode = (node, className) => {
  node.properties.class = className;
};
var addChildNode = (node, child) => {
  node.children.push(child);
};
var addTextNode = (node, text) => {
  node.children.push(createTextNode(text));
};

terser output or error

       teleport_plugin_common_dist_esm_export(_N, {
            addAttributeToNode: ()=>addAttributeToNode,
            addBooleanAttributeToNode: ()=>addBooleanAttributeToNode,
            addChildNode: ()=>addChildNode,
            addClassToNode: ()=>addClassToNode,
            addTextNode: ()=>addTextNode
        });
        for (var yN, vN, addBooleanAttributeToNode = (pT,xT,OT=!0)=>{
            pT.properties[xT] = !0 === OT && ""
        }
        , addAttributeToNode = (pT,xT,OT)=>{
            pT.properties[xT] = OT
        }
        , addClassToNode = (pT,xT)=>{
            pT.properties.class = xT
        }
        , addChildNode = (pT,xT)=>{
            pT.children.push(xT)
        }
        , addTextNode = (pT,xT)=>{
            pT.children.push(createTextNode(xT))
        }

I Recorded a small video on the behaviour and it can be found from the initial issue which i raised here webpack-contrib/terser-webpack-plugin#410 in the webpack plugin itself.

Screen.Recording.2021-07-01.at.11.30.01.PM.mov

Git Repository to reproduce the bug
https://github.com/JayaKrishnaNamburu/terser-minifier-bug-reproduction

Steps to reproduce from the Git repository
https://github.com/JayaKrishnaNamburu/terser-minifier-bug-reproduction#readme

[JayaKrishnaNamburu]

Cli version 5.7.0

I was able to reproduce this outside webpack, by using terser-cli. Here is the un-optimised chunk
https://drive.google.com/file/d/1ftvrauvf2nUITcWLchzv_DJoFQqPRTIz/view?usp=sharing

Here is the flags to be used in cli to get a chunk that generates the invalid syntax.

terser 380.7ed8f3df.js<chunk-name> -o compress.js -c ecma=5 --keep_classnames --keep_fnames

PS: The ecma=5 is important since that what causes the issue, if we remove ecma=5 then everything works just fine. But i think, the provided chunk might have some bundled es6 stuff too. Is that's what causing the breaking syntax ?

Then load the chunk in browser

[fabiosantoscode]

Ok, I got to the bottom of this.

With terser input.js -c and input:

var var1 = somethingUnknown1;
var var2 = (key) => key in somethingUnknown2

for (var i = 0; i < chars.length; i++) {
    loopyLoop()
}

Terser will join var1, var2 and i into just one decl to save space.

for (var var1 = somethingUnknown1, var2 = key => key in somethingUnknown2, i = 0; i < chars.length; i++) loopyLoop();

Because of key in somethingUnknown2 is there, it mixes up the parser, which assumed the function was just key => key and then it sees in, assuming it's a for-in loop.

[fabiosantoscode]

In the end it was just a matter of adding parens. This was already being done for other expressions, but not arrow functions.