isType

function isType(type) {
    return function (obj) {
        return {}.toString.call(obj) == '[object ' + type + ']';
    };
}
var isFunction = isType('Function');

function isType(source) {
    const target = Object.prototype.toString.call(source);
    switch (target) {
        case '[object Null]':
            return 'null';
        case '[object Undefined]':
            return 'undefined';
        case '[object String]':
            return 'string';
        case '[object Number]':
            return 'number';
        case '[object Boolean]':
            return 'boolean';
        case '[object Object]':
            return 'object';
        case '[object Array]':
            return 'array';
        case '[object Function]':
            return 'function';
        case '[object Date]':
            return 'date';
        case '[object RegExp]':
            return 'regexp';
        case '[object Error]':
            return 'error';
    }
}

function getType(target) {
    return Object.prototype.toString.call(target);
}

function typeOf(obj) {
    let res = Object.prototype.toString.call(obj).split(' ')[1];
    res = res.substring(0, res.length - 1).toLowerCase();
    return res;
}
typeOf([]); // array
typeOf({}); // object
typeOf(new Date()); // date