Slice function of JS handwriting function (thoroughly understand the header but not the footer)

One 、slice Function introduction

• slice The function is used to intercept arrays and strings
• Parameters ：start、end
• start The parameter is the interception start position ,end Is the end position of interception . Both parameters are optional .
  • When both parameters are not passed , Intercept all
  • When only one is passed start when , from start From beginning to end
• Parameters can also be negative , Many people on the Internet say counting from the back , The most correct method is negative + length , Make it a positive number .

Two 、 Handwriting slice

• Array slice

 Array.prototype.myslice =  function(start,end){
    
 	// Judge whether the parameter is negative 
    if(start&&end){
    			
        start = start<0?start = this.length+start:start
        end = end<0?end = this.length+end:end
    }
    if(!start) start = 0    	
    if(!end) end = this.length      Judge whether the parameter is transmitted 
    let res=[];
   
    for(let i=start;i<end;i++){
    
        res.push(this[i])
    }

    return res
}


const arr = [1,2,3,4,5,6,7,8,9]

console.log(arr.myslice(4,7))
console.log(arr.myslice(-4,-2))
console.log(arr.myslice(2))

• character string slice

With array slice Almost the same

String.prototype.myslice =  function(start,end){
    
    if(start&&end){
    
        start = start<0?start = this.length+start:start
        end = end<0?end = this.length+end:end
    }
    if(!start) start = 0
    if(!end) end = this.length
    let res =''
   
    for(let i=start;i<end;i++){
    
        res +=this[i]
    }

    return res
}

const str = 'hello javascript'

console.log(str.myslice(4,9))
console.log(str.myslice(-6,-3))
console.log(str.myslice(3))

3、 ... and 、 summary

slice Usage rule
1、 When both parameters are positive ： The head is not the tail
2、 When both parameters are negative ： Two parameters plus length Become a positive number , No matter how long it takes, no matter how long it takes .( Another way is to wrap the tail without wrapping the head )
3、 When two parameters are positive and negative , Better add length It is easiest to judge when it becomes positive .

• train

const str = 'hello javascript'

console.log(str.myslice(-4,9))
console.log(str.myslice(-6,-3))
console.log(str.myslice(-2))