JS type of a value

console.log(Object.prototype.toString.call(undefined))     // [object Undefined]
console.log(Object.prototype.toString.call('undefined'))   // [object String]
console.log(Object.prototype.toString.call(''))            // [object String]
console.log(Object.prototype.toString.call(4))             // [object Number]
console.log(Object.prototype.toString.call(NaN))           // [object Number]
console.log(Object.prototype.toString.call(true))          // [object Boolean]
console.log(Object.prototype.toString.call({}))            // [object Object]
console.log(Object.prototype.toString.call([]))            // [object Array]
console.log(Object.prototype.toString.call(function() {})) // [object Function]
console.log(Object.prototype.toString.call(/^[0-9]*$/))    // [object RegExp]