Do you really know JavaScript ?
这是一些网上广为传播的关于 Javascript 方面的小题，好多题还是相当有深度的，在这里汇总一下，便于学习和研究。
这些题主要针对 ECMA 262 (5.1) 的浏览器环境，如果你使用的是 Node 或是其他环境，可能表现会有点不一样。例如 this 和 global 在火狐浏览器控制台和 Node 环境中是不一样的。
本人基于自己的理解和网上的资料，整理了一份 参考答案和解析。
["1", "2", "3"].map(parseInt)
- ["1", "2", "3"]
- [1, 2, 3]
- [0, 1, 2]
- other
[typeof null, null instanceof Object]
- ["object", false]
- [null, false]
- ["object", true]
- other
[ [3,2,1].reduce(Math.pow), [].reduce(Math.pow) ]
- an error
- [9, 0]
- [9, NaN]
- [9, undefined]
var val = 'smtg';
console.log('Value is ' + (val === 'smtg') ? 'Something' : 'Nothing');
- Value is Something
- Value is Nothing
- NaN
- other
var name = 'World!';
(function () {
if (typeof name === 'undefined') {
var name = 'Jack';
console.log('Goodbye ' + name);
} else {
console.log('Hello ' + name);
}
})();
- Goodbye Jack
- Hello Jack
- Hello undefined
- Hello World
var END = Math.pow(2, 53);
var START = END - 100;
var count = 0;
for (var i = START; i <= END; i++) {
count++;
}
console.log(count);
- 0
- 100
- 101
- other
var ary = [0,1,2];
ary[10] = 10;
ary.filter(function(x) {
return x === undefined;
});
- [undefined × 7]
- [0, 1, 2, 10]
- []
- [undefined]
var two = 0.2
var one = 0.1
var eight = 0.8
var six = 0.6
[two - one == one, eight - six == two]
- [true, true]
- [false, false]
- [true, false]
- other
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 A
- Case B
- Do not know!
- undefined
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 A
- Case B
- Do not know!
- undefined
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]
parseInt(3, 8)
parseInt(3, 2)
parseInt(3, 0)
- 3, 3, 3
- 3, 3, NaN
- 3, NaN, NaN
- other
Array.isArray( Array.prototype )
- true
- false
- error
- other
var a = [0];
if ([0]) {
console.log(a == true);
} else {
console.log("wut");
}
- true
- false
- "wut"
- other
[]==[]
- true
- false
- error
- other
'5' + 3
'5' - 3
- "53", 2
- 8, 2
- error
- other
1 + - + + + - + 1
- 2
- 1
- error
- other
var ary = Array(3);
ary[0]=2
ary.map(function(elem) {
return '1';
});
- [2, 1, 1]
- ["1", "1", "1"]
- [2, "1", "1"]
- other
function sidEffecting(ary) {
ary[0] = ary[2];
}
function bar(a,b,c) {
c = 10
sidEffecting(arguments);
return a + b + c;
}
bar(1,1,1)
- 3
- 12
- error
- other
var a = 111111111111111110000,
b = 1111;
a + b;
- 111111111111111111111
- 111111111111111110000
- NaN
- Infinity
var x = [].reverse;
x();
- []
- undefined
- error
- window
Number.MIN_VALUE > 0
- false
- true
- error
- other
[1 < 2 < 3, 3 < 2 < 1]
- [true, true]
- [true, false]
- error
- other
// the most classic wtf
2 == [[[2]]]
- true
- false
- undefined
- other
3.toString()
3..toString()
3...toString()
- "3", error, error
- "3", "3.0", error
- error, "3", error
- other
(function(){
var x = y = 1;
})();
console.log(y);
console.log(x);
- 1, 1
- error, error
- 1, error
- other
var a = /123/,
b = /123/;
a == b
a === b
- true, true
- true, false
- false, false
- other
var a = [1, 2, 3],
b = [1, 2, 3],
c = [1, 2, 4]
a == b
a === b
a > c
a < c
- false, false, false, true
- false, false, false, false
- true, true, false, true
- other
var a = {}, b = Object.prototype;
[a.prototype === b, Object.getPrototypeOf(a) === b]
- [false, true]
- [true, true]
- [false, false]
- other
function f() {}
var a = f.prototype, b = Object.getPrototypeOf(f);
a === b
- true
- false
- null
- other
function foo() { }
var oldName = foo.name;
foo.name = "bar";
[oldName, foo.name]
- error
- ["", ""]
- ["foo", "foo"]
- ["foo", "bar"]
"1 2 3".replace(/\d/g, parseInt)
- "1 2 3"
- "0 1 2"
- "NaN 2 3"
- "1 NaN 3"
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", error
- other
var lowerCaseOnly = /^[a-z]+$/;
[lowerCaseOnly.test(null), lowerCaseOnly.test()]
- [true, false]
- error
- [true, true]
- [false, true]
[,,,].join(", ")
- ", , , "
- "undefined, undefined, undefined, undefined"
- ", , "
- ""
var a = {class: "Animal", name: 'Fido'};
a.class
- "Animal"
- Object
- an error
- other
var a = new Date("epoch")
- Thu Jan 01 1970 01:00:00 GMT+0100 (CET)
- current time
- error
- other
var a = Function.length,
b = new Function().length
a === b
- true
- false
- error
- other
var a = Date(0);
var b = new Date(0);
var c = new Date();
[a === b, b === c, a === c]
- [true, true, true]
- [false, false, false]
- [false, true, false]
- [true, false, false]
var min = Math.min(), max = Math.max()
min < max
- true
- false
- error
- other
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]
- [true, true]
- [false, false]
- [true, false]
- [false, true]
var a = new Date("2014-03-19"),
b = new Date(2014, 03, 19);
[a.getDay() === b.getDay(), a.getMonth() === b.getMonth()]
- [true, true]
- [true, false]
- [false, true]
- [false, false]
if ('http://giftwrapped.com/picture.jpg'.match('.gif')) {
'a gif file'
} else {
'not a gif file'
}
- 'a gif file'
- 'not a gif file'
- error
- other
function foo(a) {
var a;
return a;
}
function bar(a) {
var a = 'bye';
return a;
}
[foo('hello'), bar('hello')]
- ["hello", "hello"]
- ["hello", "bye"]
- ["bye", "bye"]
- other
var value1 = 0, value2 = 0, value3 = 0;
for ( var i = 1; i <= 3; i++) {
var i2 = i;
(function() {
var i3 = i;
setTimeout(function() {
value1 += i;
value2 += i2;
value3 += i3;
}, 1);
})();
}
setTimeout(function() {
console.log(value1, value2, value3);
}, 100);