# **==小小字串篇==**
toUpperCase --- 字串變大寫
toLowerCase --- 字串變小寫

    let str = 'Hi Im David Lee'
    let Upstr = str.toUpperCase()
    let Lowstr = str.toLowerCase()
    console.log(Upstr) // HI IM DAVID LEE
    console.log(Lowstr) // hi im david lee

substring / slice --- 字串切割 / 或字串擷取

    let str = 'HiImDavid'
    let ImDavid = str.substring(2, 9)
    let Im = str.slice(2, 4)
    console.log(ImDavid)// ImDavid
    console.log(Im)// Im

length 字串長度

    let str = 'HiImDavid'
    console.log(`${str.length}`) // 9

concat 字串連接，並返回一個新字串

    let str = 'Hi'
    let str2 = 'ImDavid'
    console.log(str.concat(str2)) // HiImDavid

split 字串切割變陣列

    let str = 'HiImDavid'
    console.log(`${str.split('')}`)
    //["H", "i", "I", "m", "D", "a", "v", "i", "d"]

trim 字串去頭去尾空白

    let str = '    HiImDavid      '
    console.log(`${str.trim()}`) // HiImDavid