详细介绍44 个 JavaScript 变态题解析

["1", "2", "3"].map(parseInt)

parseInt('1', 0);
parseInt('2', 1);
parseInt('3', 2);

[typeof null, null instanceof Object]

type         result
Undefined   "undefined"
Null        "object"
Boolean     "boolean"
Number      "number"
String      "string"
Symbol      "symbol"
Host object Implementation-dependent
Function    "function"
Object      "object"

[ [3,2,1].reduce(Math.pow), [].reduce(Math.pow) ]

var val = 'smtg';
console.log('Value is ' + (val === 'smtg') ? 'Something' : 'Nothing');

var name = 'World!';
(function () {
    if (typeof name === 'undefined') {
        var name = 'Jack';
        console.log('Goodbye ' + name);
    } else {
        console.log('Hello ' + name);
    }
})();

var name = 'World!';
(function () {
    var name;
    if (typeof name === 'undefined') {
        name = 'Jack';
        console.log('Goodbye ' + name);
    } else {
        console.log('Hello ' + name);
    }
})();

var END = Math.pow(2, 53);
var START = END - 100;
var count = 0;
for (var i = START; i <= END; i++) {
    count++;
}
console.log(count);

var ary = [0,1,2];
ary[10] = 10;
ary.filter(function(x) { return x === undefined;});

if (!Array.prototype.filter) {
  Array.prototype.filter = function(fun/*, thisArg*/) {
    'use strict';

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun !== 'function') {
      throw new TypeError();
    }

    var res = [];
    var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
    for (var i = 0; i < len; i++) {
      if (i in t) { // 注意这里!!!
        var val = t[i];
        if (fun.call(thisArg, val, i, t)) {
          res.push(val);
        }
      }
    }

    return res;
  };
}

0 in ary; => true
3 in ary; => false
10 in ary; => true

var two   = 0.2
var one   = 0.1
var eight = 0.8
var six   = 0.6
[two - one == one, eight - six == two]

function showCase(value) {
    switch(value) {
    case 'A':
        console.log('Case A');
        break;
    case 'B':
        console.log('Case B');
        break;
    case undefined:
        console.log('undefined');
        break;
    default:
        console.log('Do not know!');
    }
}
showCase(new String('A'));

var s_prim = 'foo';
var s_obj = new String(s_prim);

console.log(typeof s_prim); // "string"
console.log(typeof s_obj);  // "object"
console.log(s_prim === s_obj); // false

function showCase2(value) {
    switch(value) {
    case 'A':
        console.log('Case A');
        break;
    case 'B':
        console.log('Case B');
        break;
    case undefined:
        console.log('undefined');
        break;
    default:
        console.log('Do not know!');
    }
}
showCase2(String('A'));

function isOdd(num) {
    return num % 2 == 1;
}
function isEven(num) {
    return num % 2 == 0;
}
function isSane(num) {
    return isEven(num) || isOdd(num);
}
var values = [7, 4, '13', -9, Infinity];
values.map(isSane);

7 % 2 => 1
4 % 2 => 0
'13' % 2 => 1
-9 % % 2 => -1
Infinity % 2 => NaN

parseInt(3, 8)
parseInt(3, 2)
parseInt(3, 0)

Array.isArray( Array.prototype )

var a = [0];
if ([0]) {
  console.log(a == true);
} else {
  console.log("wut");
}

[]==[]

'5' + 3
'5' - 3

> '5' + 3
'53'
> 5 + '3'
'53'
> 5 - '3'
2
> '5' - 3
2
> '5' - '3'
2

1 + - + + + - + 1

1 + (a)  => 2
a = - (b) => 1
b = + (c) => -1
c = + (d) => -1
d = + (e) => -1
e = + (f) => -1
f = - (g) => -1
g = + 1   => 1

var ary = Array(3);
ary[0]=2
ary.map(function(elem) { return '1'; });

Array.prototype.map = function(callback, thisArg) {

        var T, A, k;

        if (this == null) {
            throw new TypeError(' this is null or not defined');
        }

        var O = Object(this);
        var len = O.length >>> 0;
        if (typeof callback !== 'function') {
            throw new TypeError(callback + ' is not a function');
        }
        if (arguments.length > 1) {
            T = thisArg;
        }
        A = new Array(len);
        k = 0;
        while (k < len) {
            var kValue, mappedValue;
            if (k in O) {
                kValue = O[k];
                mappedValue = callback.call(T, kValue, k, O);
                A[k] = mappedValue;
            }
            k++;
        }
        return A;
    };

function sidEffecting(ary) {
  ary[0] = ary[2];
}
function bar(a,b,c) {
  c = 10
  sidEffecting(arguments);
  return a + b + c;
}
bar(1,1,1)

function sidEffecting(ary) {
  ary[0] = ary[2];
}
function bar(a,b,c=3) {
  c = 10
  sidEffecting(arguments);
  return a + b + c;
}
bar(1,1,1)

var a = 111111111111111110000,
    b = 1111;
a + b;

var x = [].reverse;
x();

Number.MIN_VALUE > 0

[1 < 2 < 3, 3 < 2 < 1]

 1 < 2 => true;
 true < 3 =>  1 < 3 => true;
 3 < 2 => false;
 false < 1 => 0 < 1 => true;

// the most classic wtf
2 == [[[2]]]

3.toString()
3..toString()
3...toString()

var a = 3;
a.toString()

(function(){
  var x = y = 1;
})();
console.log(y);
console.log(x);

var a = /123/,
    b = /123/;
a == b
a === b

var a = [1, 2, 3],
    b = [1, 2, 3],
    c = [1, 2, 4]
a ==  b
a === b
a >   c
a <   c

var a = {}, b = Object.prototype;
[a.prototype === b, Object.getPrototypeOf(a) === b]

function f() {}
var a = f.prototype, b = Object.getPrototypeOf(f);
a === b

a === Object.getPrototypeOf(new f()) // true
b === Function.prototype // true

function foo() { }
var oldName = foo.name;
foo.name = "bar";
[oldName, foo.name]

"1 2 3".replace(/\d/g, parseInt)

parseInt('1', 0)
parseInt('2', 2)
parseInt('3', 4)

function f() {}
var parent = Object.getPrototypeOf(f);
f.name // ?
parent.name // ?
typeof eval(f.name) // ?
typeof eval(parent.name) //  ?

var lowerCaseOnly =  /^[a-z]+$/;
[lowerCaseOnly.test(null), lowerCaseOnly.test()]

[,,,].join(", ")

var a = {class: "Animal", name: 'Fido'};
a.class

var a = new Date("epoch")

var a = Function.length,
    b = new Function().length
a === b

var a = Date(0);
var b = new Date(0);
var c = new Date();
[a === b, b === c, a === c]

var min = Math.min(), max = Math.max()
min < max

function captureOne(re, str) {
  var match = re.exec(str);
  return match && match[1];
}
var numRe  = /num=(\d+)/ig,
    wordRe = /word=(\w+)/i,
    a1 = captureOne(numRe,  "num=1"),
    a2 = captureOne(wordRe, "word=1"),
    a3 = captureOne(numRe,  "NUM=2"),
    a4 = captureOne(wordRe,  "WORD=2");
[a1 === a2, a3 === a4]

var myRe = /ab*/g;
var str = 'abbcdefabh';
var myArray;
while ((myArray = myRe.exec(str)) !== null) {
  var msg = 'Found ' + myArray[0] + '. ';
  msg += 'Next match starts at ' + myRe.lastIndex;
  console.log(msg);
}
// Found abb. Next match starts at 3
// Found ab. Next match starts at 9

var a = new Date("2014-03-19"),
    b = new Date(2014, 03, 19);
[a.getDay() === b.getDay(), a.getMonth() === b.getMonth()]

a.getDay()
3
b.getDay()
6
a.getMonth()
2
b.getMonth()
3

if ('http://giftwrapped.com/picture.jpg'.match('.gif')) {
  'a gif file'
} else {
  'not a gif file'
}

function foo(a) {
    var a;
    return a;
}
function bar(a) {
    var a = 'bye';
    return a;
}
[foo('hello'), bar('hello')]