function Dog() {}
var obj = new Dog;
typeof obj == 'Dog' // false, typeof obj is actually "object"
obj instanceof Dog // true, what we want in this case
var subject = new Array();
var iterations = 10000000;
var goBenchmark = function(callback, iterations) {
var start = Date.now();
for (i=0; i < iterations; i++) { var foo = callback(); }
var end = Date.now();
var seconds = parseFloat((end-start)/1000).toFixed(2);
console.log(callback.name+" took: "+ seconds +" seconds.");
return seconds;
}
// Testing instanceof
var iot = goBenchmark(function instanceofTest(){
(subject instanceof Array);
}, iterations);
// Testing typeof
var tot = goBenchmark(function typeofTest(){
(typeof subject == "object");
}, iterations);
var r = new Array(iot,tot).sort();
console.log("Performance ratio is: "+ parseFloat(r[1]/r[0]).toFixed(3));
结果
instanceofTest took: 9.98 seconds.
typeofTest took: 8.33 seconds.
Performance ratio is: 1.198
let num = new Number(5);
console.log(num instanceof Number); // print true
console.log(typeof num); // print object
num++; //num is object right now but still can be handled as number
//and after that:
console.log(num instanceof Number); // print false
console.log(typeof num); // print number
function Class() {};
Class.prototype = Function;
var funcWannaBe = new Class;
console.log(funcWannaBe instanceof Function); //true
console.log(typeof funcWannaBe === "function"); //false
funcWannaBe(); //Uncaught TypeError: funcWannaBe is not a function
var f = Object.create(Function);
console.log(f instanceof Function); //=> true
console.log(typeof f === 'function'); //=> false
f(); // throws TypeError: f is not a function
var MyFunc2 = new Function('a','b','return a+b') // A function constructor is used
typeof MyFunc2 == 'function' // true
MyFunc2 instanceof Function // true
function isFunction(functionItem) {
return typeof(functionItem) == 'function' || functionItem instanceof Function;
}
isFunction(MyFunc) // invoke it by passing our test function as parameter
Muhammad Umer也在这里的一个评论中简要地提到了这一点。它适用于所有有原型的东西——所以所有不是null或undefined的东西:
// (null).constructor; // TypeError: null has no properties
// (undefined).constructor; // TypeError: undefined has no properties
(1).constructor; // function Number
''.constructor; // function String
([]).constructor; // function Array
(new Uint8Array(0)).constructor; // function Uint8Array
false.constructor; // function Boolean()
true.constructor; // function Boolean()
(Symbol('foo')).constructor; // function Symbol()
// Symbols work, just remember that this is not an actual constructor:
// new Symbol('foo'); //TypeError: Symbol is not a constructor
Array.prototype === window.frames.Array; // false
Array.constructor === window.frames.Array.constructor; // true
let s = 0,
n = 0;
function typeofSwitch(t) {
switch (typeof t) {
case "string":
return ++s;
case "number":
return ++n;
default:
return 0;
}
}
// note: no test for null or undefined here
function constructorSwitch(t) {
switch (t.constructor) {
case String:
return ++s;
case Number:
return ++n;
default:
return 0;
}
}
let vals = [];
for (let i = 0; i < 1000000; i++) {
vals.push(Math.random() <= 0.5 ? 0 : 'A');
}
typeof 'bla' === 'string';
// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean(true) === 'object';
typeof new Number(1) === 'object';
typeof new String('abc') === 'object';
const a = "I'm a string primitive";
const b = new String("I'm a String Object");
a instanceof String; --> returns false
b instanceof String; --> returns true
