let a = null
let b = undefined
let c = 0
let d = {}
console.log(typeof a, !a, a !==undefined, a !== 0)
console.log(typeof b, !b, b !== undefined, b !== 0)
console.log(typeof c, !c, c !== undefined, c !== 0)
console.log(typeof d, !d, !d.d, typeof d.d === undefined, typeof d.d === 'undefined', d.d === undefined, d.d === 'undefined')