2019/9/5 第五部分：?频考点 | FE-Interview typeof instanceof null typeof typeof 1 // number typeof 1 // string typeof undefined // undefined typeof true // boolean typeof Symbol() // symbol typeof object typeof typeof [] // object typeof {} // object typeof console.log // function instanceof const Person = function() {} const p1 = new Person() p1 instanceof Person // true var str = hello world str instanceof String // false blog.poetries.top/FE-Interview-Questions/excellent/#_36-5-动态规划 1/166 2019/9/5 第五部分：?频考点 | FE-Interview var str1 = new String(hello world) str1 instanceof String // true instanceof JS undefined null false NaN 0 -0 true [[ToPrimitive]] x.valueOf() x.toString() Symbol.toPrimitive let a = { valueOf() { blog.poetries.top/FE-Interview-Questions/excellent/#_36-5-动态规划 2/166 2019/9/5 第五部分：?频考点 | FE-Interview return 0 }, toString() { return 1 }, [Symbol.toPrimitive]() { return 2 } } 1 + a // => 3 1 + 1 // 11 true + true // 2 4 + [1,2,3] // "41,2,3" 1 11 true 1 toString 1,2,3 41,2,3 a + + b a + + b // -> "aNaN" + b NaN "aNaN" + 1 number 4 * 3 // 12 4 * [] // 0 4 * [1, 2] // NaN blog.poetries.top/FE-Interview-Questions/excellent/#_36-5-动态规划 3/166 2019/9/5 第五部分：?频考点 | FE-Interview toPrimitive unicode let a = { valueOf() { return 0 }, toString() { return 1 } } a > -1 // true a valueOf function foo() { console.log(this.a) } var a = 1 foo() const obj = { a: 2, foo: foo } obj.foo() const c = new foo() foo foo this window blog.poetries.top/FE