trim 函数

function myTrim(str) {//记录前后空格的个数,最后对字符串进行截取
    let first = 0, last = str.length
    for (let i in str) {
        if (str[i] === ' ') {
            first++
        } else {
            break
        };
    };
    for (let i = last; i > first; i--) {
        if (str[i] === ' ') {
            last--
        } else {
            break
        };
    };
    return str.substr(first, last - first)
}