js
function getDateByYYYYMMDD(yyyymmdd) {
    var tmp = void 0,
        match = void 0;
    if (match = yyyymmdd.match(/[\d]{4}\-[\d]{2}\-[\d]{2}/)) {
        if (match.length) {
            tmp = match[0].split('-');
        }
    } else if (match = yyyymmdd.match(/[\d]{4}[\d]{2}[\d]{2}/)) {
        if (match.length) {
            tmp = [match[0].substr(0, 4), match[0].substr(4, 2), match[0].substr(6, 2)];
        }
    }
    if (!tmp || tmp.length < 3) {
        return null;
    }
    return new Date(Number(tmp[0]), Number(tmp[1]) - 1, Number(tmp[2]), 0, 0, 0, 0);
}

es6
getDateByYYYYMMDD(yyyymmdd){
    let tmp,match;
    if(match = yyyymmdd.match(/[\d]{4}\-[\d]{2}\-[\d]{2}/)){
        if(match.length){
            tmp = match[0].split('-');
        }
    }else if(match = yyyymmdd.match(/[\d]{4}[\d]{2}[\d]{2}/)){
        if(match.length){
            tmp = [match[0].substr(0,4),match[0].substr(4,2),match[0].substr(6,2)];
        }
    }
    if(!tmp || tmp.length < 3){
        return null;
    }
    return new Date(Number(tmp[0]),Number(tmp[1])-1,Number(tmp[2]),0,0,0,0);
}