字符串方法

        const firstName = 'Leo';
        const lastName = 'Chen';
        const age = 24;
        const str = '低质量的社交 不如高质量的独处.愿你不再惧怕孤独 不再被孤独驱使着去找不孤独的群体.';
        const tags = '文学,科学,社会学';
  	
	let val;

拼接

        // concat 拼接
        val = firstName.concat(' ', lastName);

大小写转换

	val = firstName.toUpperCase();
	val = lastName.toLowerCase();

查找索引

 	//向前查找
	val = firstName.indexOf('o');
    	//向后查找
	val = firstName.lastIndexOf('C');
    	// 索引查找
	val = firstName.charAt('2');
    	// 获取最后一个字符
	val = firstName.charAt(firstName.length - 1);

获取子字符串

	val = firstName.substring(0, 4);

切割

	//向前切割
	val = firstName.slice(0, 4);
	//向后切割
	val = firstName.slice(-3);

转数组

	val = str.split('.');
	val = tags.split(',');