前端面试题总结

前端水很深,但我也要试着趟一趟,珍藏了一些易混淆的面试题,避免再次入坑.

js

var和let的使用

`var a = 200;
function test1 () {

console.log(a); //undefined
var b = a*2;
var a = 100;
var c = a/2;
console.log(a+","+b+","+c); //100,NaN,50

}
test1();`

`var a = 200;
function test2 () {

console.log(a); //200
var b = a*2;
a = 100;
var c = a/2;
console.log(a+","+b+","+c); //100,400,50

}
test2();`

`var a = 200;
function test3 () {

console.log(a); //报错, a is not defined
var b = a*2;
let a = 100;
var c = a/2;
console.log(a+","+b+","+c);

}
test3();`
快级作用域和变量提升问题.

下面一段代码输出什么

`var b = 10;
function t () {
```
console.log(b);
```
}
(function(fun) {
```
var b = 20;
fun();  //10
```
})(t);`

`var b = 10;
function t () {
```
console.log(b);
```
}
(function(fun) {
```
b = 20;
fun();  //20
```
})(t);`

排序算法
快排:

`function quickSort (arr) {

if (arr.length===0) {
    return [];
}

let cIndex = Math.floor(arr.length / 2),
    c = arr.splice(cIndex, 1)[0],
    l = [],
    r = [];

for (var i = arr.length - 1; i >= 0; i--) {
    if (arr[i] < c) {
        l.push(arr[i]);
    }else{
        r.push(arr[i]);
    }       
}
return quickSort(l).concat(c,quickSort(r));

}冒泡:function bubbleSort (arr) {

for (var i = 0; i< arr.length; i++) {
    for (var j = 0; j < arr.length - 1 - i; j++) {
        if (arr[j] > arr[j+1]) {
            var temp = arr[j+1];        //元素交换
            arr[j+1] = arr[j];
            arr[j] = temp;
        }
    }
}
return arr;

}`
//选择
function selectSort (arr) {
var temp,minIndex;
for(var i=0;i<arr.length; i++){

minIndex = i;
for(var j = i;j<arr.length; j++){
    if (arr[j]<arr[minIndex]) {
        minIndex = j;
    }
}
[arr[i],arr[minIndex]] = [arr[minIndex],arr[i]];
// temp = arr[i];
// arr[i] = arr[minIndex];
// arr[minIndex] = temp;

}
return arr;
}

数组生成随机数:
Array.prototype.range = function ( start,end ){
var _self = this;
var length = end - start +1;
var step = start - 1;
return Array.apply(null,{length:length}).map(function (v,i){step++;return step;});
};

ES6中class的使用
class Person{
get props(){
```
return "props";
```
}
set props(props){
```
console.log('setter: '+props);
```
}
constructor(name,age){
```
this.name = name;
this.age = age;
```
}
say(){
```
return this.name+","+this.age;
```
}
grade(){}
}
class Student extends Person{
constructor(name,age,grade){
```
super(name,age);
this.grade = grade;
```
}
say(){
```
return this.name+","+this.age+","+this.grade;
```
}
grade1(){
```
return this.grade;
```
}
}
let p = new Person(‘chao’,23);
var s = new Student(‘c’,22,98);
console.log(p.props);
p.props =”haha”;
console.log(p.props);
console.log(p.say());
console.log(s.grade1());