4 字符串的扩展 4.1 for of遍历 4.2 模板字符串 4.3 新增方法 4.3.1 String.includes() 4.3.2 String.startsWith() 4.3.3 String.endsWith() 4.3.4 String.repeat() 4.3.5 String.padStart() 4.3.6 String.padEnd() 4.3.7 String.trimStart() 4.3.8 String.trimEnd()