JavaScript基础知识篇-你真的了解JavaScript吗？

让我们重温一下JavaScript的一些基础知识，请先写出以下代码中问号处的答案，再运行比较！

<script type="text/javascript">

var a=0,b=0,c=0;
a+++b+++c;
alert([a,b,c]);//?

var a=[];
alert(a[a++]);//?
alert(a);//?

var o={a:o++};
alert(o.a);//?

var f=function(){};
alert(f(f++));

alert(typeof(NaN))//?

alert(typeof(Infinity))//?

alert(typeof(null))//?

alert(typeof(undefined))//?

alert(NaN == NaN)//?

alert(NaN != NaN)//?

alert(NaN >= NaN)//?

alert(null == undefined)//?

alert(null >= undefined)//?

alert(null <= undefined)//?

alert(parseInt("123abc"))//?

alert(("123abc" - 0))//?

alert(Infinity > 10)//?

alert(Infinity > "abc")//?

alert(Infinity == NaN)//?

alert(true == 1)//?

alert(new String("abc") == "abc")//?

alert(new String("abc") === "abc")//?

var a = "123abc";
alert(typeof(a++));//?
alert(a);//?

var a = "123abc";
a.valueOf = function(){return parseInt(a);}
alert(++a);//?
alert(a-0);//?

var a = new Object();
a.toString = function(){return "123abc";}
a.valueOf = function(){return parseInt(a);}
alert(++a);//?
alert(a-0);//?

var spv = String.prototype.valueOf;
String.prototype.valueOf = function(){
    return parseFloat(this);
}
alert("123abc" > 122);//?
alert(new String("123abc") > 122);//?
String.prototype.valueOf = spv;

var s = new String("abc");
alert(typeof(s) == typeof("abc"));//?
alert(s === "abc");//?
alert(s.toString() == s);//?

var a = new Object(),b = new Object();
a.toString = function(){return "a"};
b.toString = function(){return "b"};
alert(a>b);//?
a.valueOf = function(){return 1};
b.valueOf = function(){return 0};
alert(a>b);//?

var step = function(a){
    return function(x){
        return x + a++;
    }
}
var a = step(10);
var b = step(20);
alert(a(10));//?
alert(b(10));//?

</script>

运行代码查看结果：

<script type="text/javascript">var a=0,b=0,c=0;a+++b+++c;alert([a,b,c]);//?var a=[];alert(a[a++]);//?alert(a);//?var o={a:o++};alert(o.a);//?var f=function(){};alert(f(f++));alert(typeof(NaN))//?alert(typeof(Infinity))//?alert(typeof(null))//?alert(typeof(undefined))//?alert(NaN == NaN)//?alert(NaN != NaN)//?alert(NaN >= NaN)//?alert(null == undefined)//?alert(null >= undefined)//?alert(null <= undefined)//?alert(parseInt("123abc"))//?alert(("123abc" - 0))//?alert(Infinity > 10)//?alert(Infinity > "abc")//?alert(Infinity == NaN)//?alert(true == 1)//?alert(new String("abc") == "abc")//?alert(new String("abc") === "abc")//?var a = "123abc";alert(typeof(a++));//?alert(a);//?var a = "123abc";a.valueOf = function(){return parseInt(a);}alert(++a);//?alert(a-0);//?var a = new Object();a.toString = function(){return "123abc";}a.valueOf = function(){return parseInt(a);}alert(++a);//?alert(a-0);//?var spv = String.prototype.valueOf;String.prototype.valueOf = function(){    return parseFloat(this);}alert("123abc" > 122);//?alert(new String("123abc") > 122);//?String.prototype.valueOf = spv;var s = new String("abc");alert(typeof(s) == typeof("abc"));//?alert(s === "abc");//?alert(s.toString() == s);//?var a = new Object(),b = new Object();a.toString = function(){return "a"};b.toString = function(){return "b"};alert(a>b);//?a.valueOf = function(){return 1};b.valueOf = function(){return 0};alert(a>b);//?var step = function(a){    return function(x){        return x + a++;    }}var a = step(10);var b = step(20);alert(a(10));//?alert(b(10));//?</script>

希望大家在学习JavaScript的时候注意基本功的练习！^_^