var str="adada";console.log(str.substr(-1,2));String.prototype.substr=function(){ ????var tem=""; ???if(arguments.length===0) ???????return this.split("").join(""); ???if(arguments[0]>=this.length) ???????return tem; ???if(arguments.length===1&&arguments[0]<this.length){ ???????arguments[0]=(this.length+arguments[0])%this.length; ???????for(var i=arguments[0];i<this.length;i+=1){ ???????????tem+=this[i]; ???????} ???????return tem; ???} ???if(arguments.length===2){ ???????if(arguments[1]<=0) ???????????return tem; ???????else{ ???????????arguments[0]=(arguments[0]+this.length)%this.length; ???????????var end=arguments[0]+arguments[1]>this.length?this.length:arguments[0]+arguments[1]; ???????????for(var i=arguments[0];i<end;i+=1){ ???????????????tem+=this[i]; ???????????} ???????????return tem; ???????} ???}}console.log(str.substr(-1,2))js 模拟substr
原文地址：https://www.cnblogs.com/me-data/p/9860526.html