How does vue call the data in data in prop?

I need to set a default value in the default of prop, which is set through module css, but I found that
prop cannot call

in the document of vue.

so how can I call the value in data in prop?

prop:{
    btnStyle: {
      type: String,
      default: this.loginBtn
    }
}

data() {
    return {
         loginBtn: this.$style.loginBtn
    }
Oct.02,2021

prop:{
    btnStyle: {
      type: String,
      default: function () {
          return ...
      }
    }
}

initProps before initData , you can't get data in prop , but you can get prop

in data .
if (opts.props) initProps(vm, opts.props)
if (opts.methods) initMethods(vm, opts.methods)
if (opts.data) {
  initData(vm)
} else {
  observe(vm._data = {}, true /* asRootData */)
}
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