实现strStr()--indexOf()方法

function strStr(haystack, needle) {
    if(needle.length == 0){
        return 0
    }
    if(haystack.length == 0){
        return -1
    }
    let len = needle.length,firstChar = needle[0]
    for(let i = 0;i < haystack.length;i++){
        let item = haystack[i]
        if(item == firstChar && haystack.substr(i,len) == needle){
            return i
        }
    }
    return -1
}