字符串的新方法 - trimStart()，trimEnd()

trimStart()消除字符串头部的空格，trimEnd()消除尾部的空格

        const s = '  abc  ';

        let a = s.trim() 
        let b = s.trimStart()
        let c = s.trimEnd() 
        console.log(a)// "abc"
        console.log(b) // "abc  "
        console.log(c)// "  abc"