js对象常用方法记录

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一、join() 方法

join() 方法将数组转为字符串返回，元素将由指定的分隔符分隔，默认分隔符是逗号，不会改变原始数组。

   const months = ["January", "February", "March", "April"];
   console.log(months.join());//January,February,March,April
   console.log(months.join(""));//输出JanuaryFebruaryMarchApril
   console.log(months.join(" "));//输出January February March April
   console.log(months.join('-'));//输出January-February-March-April
   console.log(months);//输出数组["January", "February", "March", "April"]

二、shift() 方法

shift() 方法移除数组的第一项,会改变原始数组,返回值是被移除的元素。

   const months = ["January", "February", "March", "April"];
   console.log(months.shift());//输出January
   console.log(months);//输出数组['February', 'March', 'April']

三、slice() 方法

slice() 方法可从已有的数组或字符串中返回选定的元素,不会改变原始数组或字符串,以新的数组或字符串返回被提取的部分。 slice() 方法有两个参数：start和end，参数都为可选，返回的元素包括开始，但不包括结束，如果参数为负数，则表示从倒数第几个元素开始或结束，

 const months = ["January", "February", "March", "April"];
 console.log(months.slice(1,3));//输出数组['February', 'March']
 console.log(months.slice(-3,-1));//输出数组['February', 'March']
 console.log(months);//输出数组['January', 'February', 'March', 'April']

 const months = "JanuaryFebruaryMarchApril";
 console.log(months.slice(1,3));//输出an
 console.log(months.slice(-3,-1));//输出ri
 console.log(months);//输出JanuaryFebruaryMarchApril

四、substring() 方法

substring() 方法用于提取字符串中介于两个指定下标之间的字符，有两个参数from(必需，且非负，负数转化为0)和to(可选，且非负,负数转化为0，若省略该参数，默认到字符串的结尾) ，返回的字符串包括开始处的字符，但不包括结束处的字符。

const months = "JanuaryFebruaryMarchApril";
console.log(months.substring(1,3));//输出an
console.log(months.substring(2));//输出nuaryFebruaryMarchApril
console.log(months);//输出JanuaryFebruaryMarchApril

五、substr() 方法

substr()有两个参数start(必需，负数表示开始的位置为当前的负值加上字符串的长度，表示从当前下标开始截取字符串)和length(可选，非负，负数转化为0，表示需要截取的字符串的位数)，

const months = "JanuaryFebruaryMarchApril";
console.log(months.substr(1,3));//输出anu
console.log(months.substr(-3,3));//输出ril
console.log(months.substr(12));//输出aryMarchApril
console.log(months);//输出JanuaryFebruaryMarchApril