How can I ensure a function/method param is of a certian type?

Question

Is it possible to ensure that a function/method paramater is of a certain type?

For example I have a simple Character class that accepts an optional Health object. Is it possible to check that the param is infact of type Health ? I wouldn't want the consumer to pass in an integer when the application expects a Health object.

let Character = function(health) {
    if(typeof health === 'undefined')
        this.health = new Health(100);
    else
        this.health = health;
};

Character.prototype.hit = function(hitPoints) {
    this.health.subtract(hitPoints);
};

Any ideas?

Answer 1

In this particular case, yes, you have two options:

1. instanceof :

    if (health instanceof Health) { // It's a Health object *OR* a derivative of one } 

   Technically, what instanceof checks is that the object Health.prototype refers to is in health 's prototype chain.

2. Check constructor

    if (health.constructor === Health) { // Its `constructor` is `Health`, which usually (but not necessarily) // means it was constructed via Health } 

   Note that this is easy to fake: let a = {}; a.constructor = Health; let a = {}; a.constructor = Health;

Normally you'd probably want to reach for the former, because A) It allows for subtypes of Health , and B) When doing inheritance hierarchies with ES5 and earlier syntax, a lot of people forget to fix constructor and it ends up pointing to the wrong function.

Example in ES5 syntax:

 var Health = function() { }; var PhysicalHealth = function() { Health.call(this); }; PhysicalHealth.prototype = Object.create(Health.prototype); PhysicalHealth.prototype.constructor = PhysicalHealth; var h = new PhysicalHealth(); log(h instanceof Health); // true log(h.constructor == Health); // false function log(msg) { var p = document.createElement('p'); p.appendChild(document.createTextNode(msg)); document.body.appendChild(p); } 

Or using ES2015 (ES6):

 class Health { } class PhysicalHealth extends Health { } let h = new PhysicalHealth(); log(h instanceof Health); // true log(h.constructor == Health); // false function log(msg) { let p = document.createElement('p'); p.appendChild(document.createTextNode(msg)); document.body.appendChild(p); } 

Answer 2

简短的做法...

let character = function(health) { this.health = (health instanceof Health) ? health : new Health(100); }