Summary of js practical tips

Summary of js practical tips

Record the js skills used in actual combat development

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One, string, number

1. Determine how many decimal places the string (number) has

The code is as follows (example):

let x = String(value).indexOf('.')+1//小数点的位置
let y = String(value).length - x   //有几位小数

2. Limit input to only numbers

The code is as follows (react):

changeEvent=(e)=>{
    
    
    let value = e.target.value.replace(/[^\d]/, '')
    this.setState({
    
     checkCode: value })
}
 <input value={
    
    this.state.checkCode}
 	    onChange={
    
    (e) => this.changeEvent(e)>
</input>
//[^\d]表示所有除了数字以外的字符，/g表示全局匹配，
//.replace(/[^\d.]/g,"")表示将数字以外的字符替换为空

3. Number rounding

1、只取整数
parseInt(5/2)　　  // 2，丢弃小数部分,只保留整数部分
2.向上取整
Math.ceil(5/2)　　 // 3,有小数就整数部分加1
3.向下取整
Math.floor(5/2)　　// 2,丢弃小数部分
4.四舍五入
Math.round(5/2)　　// 3，四舍五入

Second, the array

1. JS deletes the first element of the array

The code is as follows (example):

arr=['George','John','Thomas']
a = arr.shift() 
//得到=》a='George'  ，arr=['John','Thomas']
//.shift() 方法用于把数组的第一个元素从其中删除，改变了原来的数组，并返回第一个元素的值
//.pop()用法和.shift()一样，把数组最后一个元素从其中删除

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2. The use of "magical oil" splice()

①Change the value of the original array without returning any value, and realize the deletion, insertion, and replacement of the array.
②The first parameter is the starting index, the second parameter is how many to replace, and the third parameter is the
code to replace. As follows (example):

插入：
var arr = ['a','b','c','d'];
arr.splice(1,0,'ttt');//第二个参数为0，代表不替换任何值     
//得到=》arr=['a','ttt','b','c','d'] 
替换：
var arr = ['a','b','c','d'];
arr.splice(1,2,'ttt');//第二个参数为2，代表替换掉两个值
console.log(arr);        
//得到=》arr=['a','ttt','c','d'] 
删除：
var arr = ['a','b','c','d'];
arr.splice(1,2);
//arr=['a','d']; 

3. Concat() connect two or more arrays

The code is as follows (example):

let a = [1,2,3];
let b = a.concat([4,5])
// 得到=>b=[1,2,3,4,5],不改变a;

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