UglifyJsPlugin : Uncaught (in promise) TypeError: Assignment to constant variable

Context

Webapp build with webpack (framework used is vuejs, from a quick look that's not relevant here)

When webapp is in dev mode, no error thrown. When webapp is in production mode (built) an error is thrown : Uncaught (in promise) TypeError: Assignment to constant variable

In store's action.js file

import Axios from 'axios';

let isError404 = function(error){
    return (
        error.status === 404 ||
        error.toString().includes('Request failed with status code 404')
    );
};

export default{
    async remoteGet (context, {url, config={}){
        return this.axios.get(url, config).catch(async (err) => {
            const error = err.response;
            if (isError404(error)) {
                return 'error'
            }
        } 
    } 
}

Quick analyse

The error seems occurs when following function is called in a get with axios.

let isError404 = function(error){
        error.status === 404 ||
        error.toString().includes('Request failed with status code 404')
}

The error does not occur anymore when this code is replaced by

const isError404 = (error) => {
        error.status === 404 ||
        error.toString().includes('Request failed with status code 404')
}

Do you have any idea why the error occurs in first place ?

EDIT : When isError404(error) is declared like : let isError404 = function(error){ ...

I don't have an error if I console.log(isError404(error)) before the if.

It seems after searching on the net that error could be cause by the package "uglifyjs-webpack-plugin": "^1.1.1", but I don't know how I can test and be sure.

Edit 2 : Webpack 3.12.0

'use strict'
const path = require('path')
const utils = require('./utils')
const webpack = require('webpack')
const config = require('../config')
const merge = require('webpack-merge')
const baseWebpackConfig = require('./webpack.base.conf')
const UglifyJsPlugin = require('uglifyjs-webpack-plugin')
const outputFile = 'fundation-app-vuejs'
const globalName = 'fundation'

const env = process.env.NODE_ENV === 'testing'
  ? require('../config/test.env')
  : require('../config/prod.env')

const webpackConfig = merge(baseWebpackConfig, {
  module: {
    rules: utils.styleLoaders({
      sourceMap: config.build.productionSourceMap,
      extract: true,
      usePostCSS: true
    })
  },
  devtool: config.build.productionSourceMap ? config.build.devtool : false,
  output: {
    path: config.build.assetsRoot,
    filename: outputFile + '.browser.js',
    library: globalName,
    libraryTarget: 'umd',
  },
  externals: [
    'axios',
    'element-ui',
    'email-validator',
    'generate-password',
    'moment',
    'schwartz',
    'v-lazy-image',
    'v-runtime-template',
    'vue',
    'vue-markdown',
    'vue-moment',
    'vue-resource',
    'vue-router',
    'vuex',
    'vuex-persistedstate',
    /^element-ui\/lib\/.*/
  ],
  plugins: [
    // http://vuejs.github.io/vue-loader/en/workflow/production.html
    new webpack.DefinePlugin({
      'process.env': env
    }),
    new UglifyJsPlugin({
      uglifyOptions: {
        compress: {
          warnings: false
        }
      },
      sourceMap: config.build.productionSourceMap,
      parallel: true
    }),
    new webpack.HashedModuleIdsPlugin(),
    new webpack.optimize.ModuleConcatenationPlugin(),
    new webpack.optimize.CommonsChunkPlugin({
      name: 'app',
      async: 'vendor-async',
      children: true,
      minChunks: 3
    }),
  ]
})

if (config.build.productionGzip) {
  const CompressionWebpackPlugin = require('compression-webpack-plugin')

  webpackConfig.plugins.push(
    new CompressionWebpackPlugin({
      asset: '[path].gz[query]',
      algorithm: 'gzip',
      test: new RegExp(
        '\\.(' +
        config.build.productionGzipExtensions.join('|') +
        ')$'
      ),
      threshold: 10240,
      minRatio: 0.8
    })
  )
}

if (config.build.bundleAnalyzerReport) {
  const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPlugin
  webpackConfig.plugins.push(new BundleAnalyzerPlugin())
}

module.exports = webpackConfig