关于 JavaScript 的36道题 [ JavaScript Puzzlers! ]
来源：互联网 发布：艾瑞数据网站排名 编辑：程序博客网 时间：2024/05/16 12:07
最近在微博上发现了一个网站 [ http://javascript-puzzlers.herokuapp.com/ ] 是关于 JavaScript 的36道题, 于是尝试自己做了下, 挺不错的, 在此总结下小小的收获. ^-^..
["1", "2", "3"].map(parseInt)["1", "2", "3"][1, 2, 3][0, 1, 2]otherwhat you actually get is [1, NaN, NaN] because parseInt takes two parameters (val, radix) and map passes 3(element, index, array)
(1) 对于无法转换的值会返回 NaN.( 例如: parseInt('2', 2); 二进制状态下只能出现0/1, 2已超出其基数范围,返回 NaN )
(2) 不提供字符串的进制或者提供字符串的进制为 0, 若字符串以'0x'开头采用十六进制,若以'0'开头采用八进制,其他开头采用十进制
[typeof null, null instanceof Object]["object", false][null, false]["object", true]othertypeof will always return "object" for native non callable objects.
[ [3,2,1].reduce(Math.pow), [].reduce(Math.pow) ]an error[9, 0][9, NaN][9, undefined]Per spec: reduce on an empty array without an initial value throws TypeError
var val = 'smtg';console.log('Value is ' + (val === 'smtg') ? 'Something' : 'Nothing');Value is SomethingValue is NothingNaNotherit actually prints 'Something' the + operator has higher precedence than the ternary one.
var name = 'World!';(function () { if (typeof name === 'undefined') { var name = 'Jack'; console.log('Goodbye ' + name); } else { console.log('Hello ' + name); }})(); Goodbye JackHello JackHello undefinedHello WorldThe var declaration is hoisted to the function scope, but the initialization is not.
var END = Math.pow(2, 53);var START = END - 100;var count = 0;for (var i = START; i <= END; i++) { count++;}console.log(count);0100101otherit goes into an infinite loop, 2^53 is the highest possible number in javascript, and 2^53+1 gives 2^53, so i can never become larger than that.
var ary = [0,1,2];ary[10] = 10;ary.filter(function(x) { return x === undefined;});[undefined × 7][0, 1, 2, 10][][undefined]Array.prototype.filter is not invoked for the missing elements.
var two = 0.2var one = 0.1var eight = 0.8var six = 0.6[two - one == one, eight - six == two][true, true][false, false][true, false]otherJavaScript does not have precision math, even though sometimes it works correctly.
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'));Case ACase BDo not know!undefinedswitch uses === internally and new String(x) !== x
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'));Case ACase BDo not know!undefinedString(x) does not create an object but does return a string, i.e. typeof String(1) === "string"
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);[true, true, true, true, true][true, true, true, true, false][true, true, true, false, false][true, true, false, false, false]Infinity % 2 gives NaN, -9 % 2 gives -1 (modulo operator keeps sign so it's result is only reliable compared to 0)
parseInt(3, 8)parseInt(3, 2)parseInt(3, 0)3, 3, 33, 3, NaN3, NaN, NaNother3 doesn't exist in base 2, so obviously that's a NaN, but what about 0? parseInt will consider a bogus radix and assume you meant 10, so it returns 3.
Array.isArray( Array.prototype )truefalseerrorotherArray.prototype is an Array. Go figure.
var a = [0];if ([0]) { console.log(a == true);} else { console.log("wut");}truefalse"wut"other[0] as a boolean is considered true. Alas, when using it in the comparisons it gets converted in a different way and all goes to hell.
[]==[]truefalseerrorother== is the spawn of satan.
'5' + 3 '5' - 3"53", 28, 2errorotherStrings know about + and will use it, but they are ignorant of - so in that case the strings get converted to numbers.
1 + - + + + - + 121errorotherGreat fun.
var ary = Array(3);ary[0]=2ary.map(function(elem) { return '1'; });[2, 1, 1]["1", "1", "1"][2, "1", "1"]otherThe result is ["1", undefined × 2], as map is only invoked for elements of the Array which have been initialized.
function sidEffecting(ary) { ary[0] = ary[2];}function bar(a,b,c) { c = 10 sidEffecting(arguments); return a + b + c;}bar(1,1,1)312errorotherThe result is 21, in javascript variables are tied to the arguments object so changing the variables changesarguments and changing arguments changes the local variables even when they are not in the same scope.
var a = 111111111111111110000, b = 1111;a + b;111111111111111111111111111111111111110000NaNInfinityLack of precision for numbers in JavaScript affects both small and big numbers.
var x = [].reverse;x();[]undefinederrorwindow[].reverse will return this and when invoked without an explicit receiver object it will default to the defaultthis AKA window
Number.MIN_VALUE > 0falsetrueerrorotherNumber.MIN_VALUE is the smallest value bigger than zero, -Number.MAX_VALUE gets you a reference to something like the most negative number.
[1 < 2 < 3, 3 < 2 < 1][true, true][true, false]errorotherImplicit conversions at work. both true and false are greater than any number.
// the most classic wtf 2 == [[[2]]]truefalseundefinedotherboth objects get converted to strings and in both cases the resulting string is "2"
3.toString()3..toString()3...toString()"3", error, error"3", "3.0", errorerror, "3", errorother3.x is a valid syntax to define "3" with a mantissa of x, toString is not a valid number, but the empty string is.
(function(){ var x = y = 1;})();console.log(y);console.log(x);1, 1error, error1, errorothery is an automatic global, not a function local one.
var a = /123/, b = /123/;a == ba === btrue, truetrue, falsefalse, falseotherPer spec Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical.
var a = [1, 2, 3], b = [1, 2, 3], c = [1, 2, 4];a == b; a === b; a > c; a < c; false, false, false, truefalse, false, false, falsetrue, true, false, trueotherArrays are compared lexicographically with > and <, but not with == and ===
var a = {}, b = Object.prototype;[a.prototype === b, Object.getPrototypeOf(a) === b][false, true][true, true][false, false]otherFunctions have a prototype property but other objects don't so a.prototype is undefined.
Every Object instead has an internal property accessible via Object.getPrototypeOf
function f() {}var a = f.prototype, b = Object.getPrototypeOf(f);a === btruefalsenullotherf.prototype is the object that will become the parent of any objects created with new f whileObject.getPrototypeOf returns the parent in the inheritance hierarchy.
function foo() { }var oldName = foo.name;foo.name = "bar";[oldName, foo.name]error["", ""]["foo", "foo"]["foo", "bar"]name is a read only property. Why it doesn't throw an error when assigned, I do not know.
"1 2 3".replace(/\d/g, parseInt)"1 2 3""0 1 2""NaN 2 3""1 NaN 3"String.prototype.replace invokes the callback function with multiple arguments where the first is the match, then there is one argument for each capturing group, then there is the offset of the matched substring and finally the original string itself. so parseInt will be invoked with arguments 1, 0, 2, 2, 3, 4.
function f() {}var parent = Object.getPrototypeOf(f); f.name // ? parent.name // ? typeof eval(f.name) // ? typeof eval(parent.name) // ? "f", "Empty", "function", "function""f", undefined, "function", error"f", "Empty", "function", errorotherThe function prototype object is defined somewhere, has a name, can be invoked, but it's not in the current scope.
var lowerCaseOnly = /^[a-z]+$/; [lowerCaseOnly.test(null), lowerCaseOnly.test()][true, false]error[true, true][false, true]the argument is converted to a string with the abstract ToString operation, so it is "null" and "undefined".
[,,,].join(", ")", , , ""undefined, undefined, undefined, undefined"", , """JavaScript allows a trailing comma when defining arrays, so that turns out to be an array of three undefined.
var a = {class: "Animal", name: 'Fido'};a.class"Animal"Objectan errorotherThe answer is: it depends on the browser. class is a reserved word, but it is accepted as a property name by Chrome, Firefox and Opera. It will fail in IE. On the other hand, everybody will accept most other reserved words (int, private, throws etc) as variable names too, while class is verboten.
- 关于 JavaScript 的36道题 [ JavaScript Puzzlers! ]
- JavaScript Puzzlers!
- JavaScript Puzzlers javascript解惑
- JavaScript Puzzlers (试题)
- javascript-puzzlers.herokuapp 刷题笔记
- 关于javascript的调试
- javascript 关于日期的
- 关于javascript的调试
- 关于javascript的基础知识
- 关于javascript的调试
- 关于javascript的疑问
- 关于javascript的字符串
- 关于Javascript的学习
- 关于JavaScript的this
- 关于javascript的继承
- 关于JavaScript 的事件
- 关于javascript的prototype
- 关于javascript的typeof
- NYOJ 663 弟弟的作业
- 32位与64位下各类型长度对比
- 第五章里KeyFile保护的实例的不解
- 下载网络文件HttpURLConnection.getContentLength()大小为 0
- n 的阶乘 C 语言版（大数）
- 关于 JavaScript 的36道题 [ JavaScript Puzzlers! ]
- 其实，不用逼迫，人们也想结婚
- preg_match_all
- 怎么查看自己匿名在百度知道提问的问题？
- KMP小结
- SQL中SELECT INTO和INSERT INTO SELECT(复制表)
- HDU1867_A + B for you again
- 搜索（bfs）
- java笔记10
