1
isNaN(NaN) // true
2
isNaN(10) // false
3
isNaN('10') // false
4
isNaN('blue') //true
5
isNaN(true) // false

1
parseInt("123") //123
2
parseInt("123abc") // 123
3
parseInt("") // NaN
4
parseInt("0xf") // 15
5
parseInt("07") // 7

1
parseInt("0xAF",16) // 175
2
parseInt("AF",16) // 175
3
parseInt("AF") //NaN

1
let num = 10;
2
num.toString(); // "10"
3
num.toString(2); // "1010"
4
num.toString(8); // "12"
5
num.toString(16); // "a"

1
class Foo{
2
    async *[Symbol.asyncIterator](){}
3
}
4
let f = new Foo();
5
console.log(f[Symbol.asyncIterator()]);
6

7
// AsyncGenerator(<suspended>)

1
class Emitter {
2
  constructor(max){
3
        this.max = max;
4
        this.index = 0;
5
    }
6
    async *[Symbol.asyncIterator](){
7
        while(this.index< this.max){
8
            yield new Promise((resolve)=>resolve (this.index++));
9
        }
10
    }
11
}
12

13
async function count(number){
14
    let emitter = new Emitter(number);
15
    for await(const x of emitter){
16
        console.log(x)
17
    }
18
}
19

20
count(5);
21
// 0
22
// 1
23
// 2
24
// 3
25
// 4
26
//

1
function Foo(){}
2
let f = new Foo();
3
console.log(f instanceof Foo); // true
4
console.log(Foo[Symbol.hasInstance](f)) // true;
5

6
class FlaseInstance(){
7
    static [Symbol.hasInstance](){
8
        return false
9
    }
10
}
11

12
let a = new FalseInstance();
13
console.log(a intanceof FalseInstance) //false

1
let init = ['foo'];
2

3
let arr = ['bar'];
4
console.log(arr[Symbol.isConcatSpreadable]) // undefined
5
console.log(init,concat(arr)) // ['foo', 'bar']
6
arr[Symbol.isConcatSpreadable] = false;
7
console.log(init.concat(arr)) // ['foo', Array(1)];
8

9
let likeObj = { length:1, 0: 'bar'};
10
console.log(likeObj[Symbol.isConcatSpreadable]); //undefined
11
console.log(init.cancat(likeObj)) // ['foo', {...}];
12
likeObj[Symbol.isConcatSpreadable] = true;
13
console.log(init.concat(likeObj)) // ['foo', 'bar']

1
class Bar {
2
  constructor(max){
3
    this.max = max;
4
    this.index = 0;
5
  }
6
  *[Symbol.iterator](){
7
    while(this.index < this.max){
8
      yield this.index++;
9
    }
10
  }
11
}
12

13
function count(){
14
  let bar = new Bar(5);
15
  for(const x of bar){
16
    console.log(x)
17
  }
18
}
19
// 0
20
// 1
21
// 2
22
// 3
23
// 4

1
console.log(RegExp.prototype[Symbol.match]);
2
// f [Symbol.match]() { [native code] }
3
console.log('foobar'.match(/bar/));
4
// ["bar", index: 3, input: "foobar", groups: undefined]

1
class FooMatcher {
2
     static [Symbol.match](target){
3
         return target.includes('foo')
4
     }
5
}
6

7
console.log('foobar'.match(FooMacther)) // true;
8

9
class StringMatcher{
10
    constructor(str){
11
        this.str = str;
12
    }
13
    [Symbol.match](target){
14
        return target.includes(this.str);
15
    }
16
}
17

18
console.log('foobar'.match(new StringMacther('foo'))) // true

1
class FooReplacer {
2
    static [Symbol.replace](target, replacememt) {
3
        return target,split('foo').join(replacement);
4
    }
5
}
6
console.log('barfoobaz'.replace(FooReplacer, 'quz'));
7
// barquxbaz
8

9
class StringReplacer {
10
    constructor(str){
11
        this.str = str;
12
    }
13
    [Symbol.replace](target, replacement){
14
        return target.split(this.str).join(replacement);
15
    }
16
}
17
console.log('barfoobaz'.replace(new StringReplacer('foo'), 'quz'));
18
// barfoobaz

1
console.log('foobar'.search(/bar/)); //3
2

3
class StringSearcher {
4
  constructor(str){
5
    this.str = str;
6
  }
7
  [Symbol.search](target){
8
    return target.indexOf(this.str);
9
  }
10
}
11

12
console.log('foobar'.search(new StringSeacher('foo'))); // 0

1
class Bar extends Array {}
2
class Baz extends Array {
3
    static get [Symbol.species](){
4
        return Array;
5
    }
6
}
7

8
let bar = new Bar();
9
console.log(bar instanceof Array) // true;
10
console.log(bar instanceof Bar) // true;
11

12
let baz = new Baz()
13
console.log(baz instanceof Array) // true
14
console.log(baz instanceof Baz) // true
15
baz = baz.concat('baz');
16
console.log(baz instanceof Array) // true
17
console.log(baz instanceof Baz) // false

1
class Foo{}
2
let foo = new Foo();
3
console.log(3 + foo); // 3[object Object]
4
console.log(3 - foo); // NaN
5
console.log(String(foo)); // [object Object]
6

7
class Baz{
8
    constructor(){
9
        this[Symbol.toPrimitive] = function(hint){
10
            switch(hint){
11
        case 'number':
12
                    return 3;
13
                case 'string':
14
                    return 'string bar';
15
                case 'default':
16
                default:
17
                    return 'default bar';
18
            }
19
        }
20
    }
21
}
22

23
let bar = new Bar();
24

25
console.log(3 + bar) // 3default bar
26
console.log(3 - bar) // 0
27
console.log(String(3)) // string bar

1
let s = new Set();
2

3
console.log(s) // Set(0){}
4
console.log(s.toString()); // [object Set];
5
console.log(s[Symbol.toStringTag]); // Set
6

7
class Baz{
8
    constructor(){
9
        this[Symbol.toStringTag] = "Bar";
10
    }
11
}
12

13
let baz = new Baz(); // Baz {}
14
console.log(baz.toString()) // [object Baz]
15
console.log(baz[Symbol.toStringTag]) // Baz