Unminified variable name used unexpectedly

minify unless you turn mangle off replaces the original variable names with one-letter names. But under some curcumstances it forgets to use the minified variable name and uses the unmodified variable name instead which leads to a ReferenceError since it’s not defined.

To Reproduce

See configuration below. This does not occur in REPL.

• Source code

String.prototype.toBlob = function() {
  try {
    let arr = this.split(',')
    , mime = arr[0].match(/:(.*?);/)[1]
    , bstr = atob(arr[1])
    , n = bstr.length
    , u8arr = new Uint8Array(n);
    while(n--){
      u8arr[n] = bstr.charCodeAt(n);
    }
    return new Blob([u8arr], {type:mime});
  }
  catch (e) {
    console.error(e)
    return false
  }
}

Actual Output

String.prototype.toBlob = function() {
  try {
    for (var a = this.split(','), b = a[0].match(/:(.*?);/)[1], c = atob(a[1]), d = c.length, e = new Uint8Array(d); d--;) 
      e[d] = c.charCodeAt(d);
    return new Blob([u8arr], { type: mime }) 
  } 
  catch (a) {
    return console.error(a), !1 } 
}

This obviously produces a ReferenceError: u8arr is not defined.

Expected Output

Here’s an actual ouput produced long time ago by "babel-cli": "6.18.0", "babel-preset-latest": "6.16.0" and "babel-preset-babili": "0.0.9"

String.prototype.toBlob = function() {
  try {
    for (var a = this.split(','), b = a[0].match(/:(.*?);/)[1], d = atob(a[1]), g = d.length, h = new Uint8Array(g); g--;) 
      h[g] = d.charCodeAt(g);
    return new Blob([h], {
      type: b
    })
  } catch (a) {
    return console.error(a), !1
  }
};

Configuration

• package.json (excerpt):

{
  "scripts": {
    "build": "cross-env BABEL_ENV=production babel src --out-dir build --source-maps"
  },
  "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-preset-env": "^1.7.0",
    "babel-preset-minify": "^0.4.3",
    "cross-env": "^5.1.6"
  }
}

• .babelrc:

{
  "presets": [["env", {"modules": false, "comments": false}]],
  "env": {
    "production": {
      "presets": ["minify"]
    }
  }
}

To build, run npm run build.

[davidbonnet]

This comment relates to previous issues with let and for loops: https://github.com/babel/minify/issues/485#issuecomment-291024057

[Cyp]

Reduced testcase Doesn’t trigger with var instead of let, and doesn’t trigger without env or simplify.

function baz() {
    let foo = 42;
    while (bar) {
    }
    return foo;
}

Actual

"use strict";function baz(){for(;bar;);return foo}

Expected

"use strict";function baz(){for(;bar;);return 42}