can't keep `const`, `let` in generate file

[bluelovers]

Current Behavior

all const let became var

Expected behavior

keep const let

Suggested solution(s)

Additional context

i also try set preferConst for rollup, but not work

// Not transpiled with TypeScript or Babel, so use plain Es6/Node.js!
module.exports = {
    // This function will run for each entry/format/env combination
    rollup(config, options) {

        console.dir([config, options]);

        config.output.preferConst = true;

        console.dir([config, options]);

        return config;
    },
};

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