notes about with statement.

with.js

const obj = {
  x: 10,
  foo: function foo () {
    function bar () {
      console.log(x) // undefined, scope foo
      console.log(y) // 30, scope bar
      console.log(this.x) // 20
      console.log(this.y) // undefined
    }

    with (this) {
      // let x; x = 20 // will not change this.x
      var x = 20 // declare x in scope foo, this.x === obj.x
      var y = 30 // declare y in scope foo, y = 30

      bar.call(this) // this === obj
    }
  }
}

obj.foo()