关于js中一些看上去简单又不会写的题

x !== x 返回true

const x = ? // Please fill in the value of "x?
if (x !== x) {
  console.log('hello fatfish')
}

如果你有仔细阅读 es6 的教程的话，就知道这个问题是一个典型的demo

const x = NaN // Please fill in the value of "x?
if (x !== x) {
  console.log('hello fatfish')
}
console.log(NaN === NaN) // false
console.log(x !== x) // true
console.log(Number.isNaN(x)) // true

(!isNan(x) && x !== x) 返回 true

现在难度提高

const x = ? // Please fill in the value of "x?
if(!isNaN(x) && x !== x) {
  console.log('hello fatfish')
}

你知道 Object.defineProperty() 吗？

请参考Object.defineProperty() - JavaScript | MDN

window.x = 0 // Any value is OK
Object.defineProperty(window, 'x', {
  get () {
    return Math.random()
  }
})
console.log(x) // 0.12259077808826002
console.log(x === x) // false
console.log(x !== x) // true

如何使得 x === x+1

使用极大值 Number.MAX_SAFE_INTERGER
参考 Number.MAX_SAFE_INTEGER - JavaScript | MDN

const x =  Number.MAX_SAFE_INTEGER + 1// Please fill in the value of "x?
if (x === x + 1) {
  console.log('hello fatfish')
}

如何使得 x > x

使用 Symbol.toPrimitive

const x = { // Please fill in the value of "x?
  value: 1,
  [ Symbol.toPrimitive ] () {
    console.log('x', this.value)
    return --this.value
  }
}

if (x > x) {
  console.log('hello fatfish')
}

实现 typeof x === ‘undefined’ && x.length > 0

答案是 document.all()

const x = document.all // Please fill in the value of "x?
if(typeof x === 'undefined' && x.length > 0) {
  console.log('hello fatfish')
}

console.log(x)
console.log(typeof x)
console.log(x === undefined)

参考 javascript - Why is document.all falsy? - Stack Overflow