webpack.config.js

var webpack = require('webpack');
var path = require('path');

module.exports = {
    entry: './test.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'my-first-webpack.bundle.js',
        library: "MyLibrary",
        libraryTarget: 'umd',
        umdNamedDefine: true
    },
    module: {
        rules: [{
            test: /\.js$/,
            loader: 'babel-loader'
        }]
    },
    plugins: [
        new webpack.optimize.UglifyJsPlugin({
            beautify: false,
            comments: false,
            compress: {
                warnings: false,
                drop_console: true,
                collapse_vars: true,
                reduce_vars: true,
            }
        })
    ]
}

test.js

let aLooooongName = 'test webpack';
console.log(123123123123123123)
// zhushihasdifhaisdhfih
if (true) {
    aLooooongName = 'true value'
} else {
    aLooooongName = 'false value'
}

export default aLooooongName;

结果经过webpack打包出来的文件并没有压缩，但是将test.js改成以下样子之后就可以压缩了，是什么情况？

var aLooooongName = 'test webpack';
console.log(123123123123123123)
// zhushihasdifhaisdhfih
if (true) {
    aLooooongName = 'true value'
} else {
    aLooooongName = 'false value'
}

export default aLooooongName;

也就将一个let变成了var了，就能压缩了什么情况？？？
求助！！！！