Day17:web前端开发面试题

我不是女神ヾ 2021-09-30 08:00 420阅读 0赞

## 1.JavaScript 数据类型有哪些？ ##

> JavaScript 变量能够保存多种数据类型：数值、字符串值、数组、对象等等：

var length = 7;                             // 数字
    var lastName = "7";                      // 字符串
    var cars = ["1", "2", "3"];         // 数组
    var x = {firstName:"1", lastName:"2"};    // 对象

复杂数据  
typeof 运算符可返回以下两种类型之一：

function  
object

typeof {name:'1', age:2} // 返回 "object"
    typeof [1,2,3,4]             // 返回 "object" 
    typeof null                  // 返回 "object"
    typeof function myFunc(){}   // 返回 "function"

原始数据

typeof 运算符可返回以下原始类型之一：

string  
number  
boolean  
undefined

> Undefined 与 Null 的区别

Undefined 与 null 的值相等，但类型不相等

typeof undefined              // undefined
    typeof null                   // object
    null === undefined            // false
    null == undefined             // true

实例

var person = null;           // 值是 null，但是类型仍然是对象

undefined 清空对象：

实例

var person = undefined;     // 值是 undefined，类型是 undefined

> 空值

空值与 undefined

var car = "";                // 值是 ""，类型是 "string"

Undefined  
在 JavaScript 中，没有值的变量，其值是 undefined。typeof 也返回 undefined。

任何变量均可通过设置值为 undefined 进行清空。其类型也将是 undefined。

typeof 运算符对数组返回 “object”，因为在 JavaScript 中数组属于对象。

JavaScript 布尔值  
布尔值只有两个值：true 或 false。

JavaScript 数组  
JavaScript 数组用方括号书写。

数组的项目由逗号分隔。

JavaScript 数值  
JavaScript 只有一种数值类型。

JavaScript 字符串值  
字符串（或文本字符串）是一串字符

> JavaScript 基本数据类型

基本数据类型有五种：字符串（String）、数字(Number)、布尔(Boolean)、空（Null）、未定义（Undefined）。

> 复合数据类型

对象(Object)、数组(Array)、函数（function）。

JavaScript基本数据类型

（1）数值类型

（2）布尔类型

（3）字符串类型

（4）Undefined类型

（5）Null类型

## 2.JavaScript 保留关键字有哪些？ ##

JavaScript 保留关键字  
Javascript 的保留关键字不可以用作变量、标签或者函数名。

abstract arguments boolean break byte  
case catch char class\* const  
continue debugger default delete do  
double else enum\* eval export\*  
extends\* false final finally float  
for function goto if implements  
import\* in instanceof int interface  
let long native new null  
package private protected public return  
short static super\* switch synchronized  
this throw throws transient true  
try typeof var void volatile  
while with yield

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JavaScript 对象、属性和方法

Array Date eval function hasOwnProperty  
Infinity isFinite isNaN isPrototypeOf length  
Math NaN name Number Object  
prototype String toString undefined valueOf

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JavaScript 所有关键字：

break  
case  
catch  
continue  
default  
delete  
do  
else  
finally  
for  
function  
if  
in  
instanceof  
new  
return  
switch  
this  
throw  
try  
typeof  
var  
void  
while  
with

> throw 与 try…catch…finally：操作异常  
> in：与for 一起使用用于遍历对象的属性名  
> instanceof ：返回的是布尔值，而typeof 返回的是几种数据类型的字符串值。

JavaScript 关键字

> JavaScript 关键字必须以字母、下划线（\_）或美元符（$）开始。

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## 3.比较表达式 ##

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## 4.判断为0 ##

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## 5.result的值 ##

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## 6.result的值 ##

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## 7.value的值 ##

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## 8. 函数值 ##

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## 9.result的值 ##

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## 10.返回前一个浏览的页面 ##

back()方法，加载 history 列表中的前一个 URL。

语法：

window.history.back();

比如，返回前一个浏览的页面，代码如下：

window.history.back();

back()相当于go(-1),代码如下:

window.history.go(-1);

在HTML文档对象模型中，history对象的（）用于加载历史列表中的下一个URL页面。

forward（）

## 11.结果 ##

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Math.floor  
document.write(Math.floor(0.6) =0  
document.write(Math.floor(1.40) =1

求一个最接近它的整数，它的值小于或等于这个浮点数

## 12.常见的HTTP状态码(HTTP Status Code) ##

常见状态吗：400（请求无效），401（需要权限），500（服务器错误）

onreadystatechange 存储函数（或函数名），每当 readyState 属性改变时，就会调用该函数。  
readyState  
存有 XMLHttpRequest 的状态。从 0 到 4 发生变化。

readyState

XMLHttpRequest对象的status属性状态吗

200 服务器正常处理了请求并响应  
404 请求的页面（资源）没有找到  
403 没有权限访问请求的页面（资源）  
405 页面（资源）不接收该请求方式  
408 请求超时  
500 服务器处理请求时遇到错误（可能因为应用程序抛出异常导致）  
502 上游服务器和网关/代理不同意的协议交换数据  
503 服务器暂时不可用

> XMLHttpRequest的属性与状态码

XMLHttpRequest对象有一个属性可以获得服务器传回的HTTP状态码

onreadystatechange:  
每次状态改变所触发事件的事件处理程序

readyState:对象状态值

responseText:从服务器进程返回的数据的字符串形式

status:从服务器返回的数字代码

responseXML:从服务器进程返回的数据的字符串形式

## 13.cookie的生存周期的关键字 ##

cookie对象的Expires属性用来设置当前cookie的存活时间。

在默认情况下，cookie是临时存在的。在一个浏览器窗口打开时，可以设置cookie，只要该浏览器窗口没有关闭，cookie就一直有效，而一旦浏览器窗口关闭后，cookie也就随之消失。

为cookie设置一个生存期

所谓生存期也就是cookie的终止日期，在这个终止日期到达之前，浏览器随时都可以读取该cookie。一旦终止日期到达之后，该cookie将会从cookie文件中删除。

## 14.数组用什么方法可以将数组中的元素连成一个字符串 ##

join() 方法用于把数组中的所有元素放入一个字符串

## 15.x值 ##

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都为undefined

## 16. 求值 ##

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## 17.数组去除数组中重复的元素 ##

Array.prototype.method1 = function() {
     var arr=[];
     for(var i = 0; i<this.length; i++) {
      if(arr.indexOf(this[i] == -1) {
       arr.push(this[i]);
      }
     }
    }

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> javaScript实现去除数组中重复的元素

1.先排序  
2.遍历数组每个元素  
3.让前一个元素与后一个元素相比较，若相等，删除前一个  
4.每次删除元素会使数组长度减一，再次循环时会少对比一个 ，所以删除后i–

var arr=[11,32,3,4,2,32,4,12,3,11,23];
    arr.sort(function(x,y){
        return x-y;
    });

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for(var i=0; i<arr.length; i++){
        if(arr[i] == arr[i+1]){
            arr.splice(i,1);
            i--;
        }
    }
    console.log(arr);

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