JavaScript精选练习（1）

var arr = [1,2];
var arr1 = arr ;
arr.push(3);
document.write(arr1)

---

答案：arr1 = [1,2,3]
( 理解stack heap原理)

var arr = [1,2];
var arr1 = arr;
arr = [1,2,3]
document.write(arr1)

---

答案：arr1 = [1,2]

var a = "10" > "8";
document.write(a)

---

答案：false
字符串比较原则 第一位先进行比较，再进行第二位比较..

var a = 1 && 2+2;
document.write(a);

---

答案：4

var a = 1 + 1 && 1 - 1;
document.write(0);

---

答案：0

打印100以内的所有质数

---

    var count = 0;
    for (i = 1; i < 100; i++) {
        for (j = 1; j <= i; j++) {
            if (i % j == 0) {
                count++;
            }
        }
        if (count == 2) {
            console.log(i + ' ')
        }
        count = 0;
    }

写出斐波那契数列

---

    var n = parseInt(window.prompt('input'));
    var first = 1;
    var second = 1;
    var third;
    if (n > 2) {
        for (i = 0; i < n - 2; i++) {
            third = first + second;
            first = second;
            second = third;
        }
        document.write(third);
    }else {
        document.write(1);
    }

    var num = null;
    console.log(typeof(num));

---

答案：object
null在最早作为空对象使用（历史遗留问题）
typeof能返回六种数据类型：
number,string,object,undefined,function,boolean

    var num = 1 + '1';
    console.log(typeof (num) + ':' + num);

---

答案：string:11
JS独有的隐式类型转换

    var num = 1 * '1';
    console.log(typeof (num) + ':' + num);

---

答案：number:1

    var num = 1 - '1';
    console.log(typeof (num) + ':' + num);

---

答案：number:0

    var a = undefined;
    var num = Number(a);
    console.log(num);

---

答案：NaN

    var num = true;
    console.log(typeof (parseInt(num)) + ' ' + parseInt(num))

---

答案：Number : NaN
Number()更关注值，parseInt（）更关注转换类型。
需要注意的是parseInt不是四舍五入，而是直接去掉小数。

    var num = parseInt(10,16);
    console.log(num);

---

答案：16
parseInt(num,radix)是以16进制为基底，将num转化为以10进制的数字

    var demo = '100px';
    console.log(parseInt(demo));

---

答案：100
parseInt读到非数字类为止

    var demo = undefined;
    var num = demo.toString();
    console.log(typeof (num) + ':' + num);

---

答案：报错
undefined与null 不能使用toString()

    var demo = 10;
    var num = demo.toString(8);
    console.log(typeof (num) + ':' + num);

---

答案：string：12
radix为8 ，以8进制为基底

    var demo = undefined;
    var num = String(demo);
    console.log(typeof (num) + ':' + num);

---

答案：string：undefined
String()方法，写什么就转化什么为字符串

console.log(isNaN("123"))

---

答案：false

console.log(isNaN("abc"))

---

答案：true

如果对于隐式类型转换不太了解可以看看：
JavaScript隐式类型转换