what is `this` in function in function

thisInFunction.js

class A {
  constructor(value) {
    this.value = value
  }

  foo() {
    const fun = function() { console.log('fun: ', this) }
    fun();

    const bindFun = function() { console.log('bindFun: ', this) }.bind(this)
    bindFun();

    const arrowFun = () => { console.log('arrowFun: ', this) }
    arrowFun()
  }
}

new A("1").foo()

// fun:  undefined
// bindFun:  A { value: '1' }
// arrowFun:  A { value: '1' }