share-property.md

Question

实例属性在原型继承中很容易被意外共享

// If property is primitive value, it will be fine.
function Klass () {}
Klass.prototype.prop = 0;
Klass.prototype.setProp = function (value) {
  this.prop = value;
};
var obj1 = new Klass();
var obj2 = new Klass();
obj2.setProp(1);
console.log(obj1.prop === 0);  // will be true
console.log(obj2.prop === 1);  // will be true

// Problem comes with non-primitive value
function $Set () {}
$Set.prototype.items = [];
$Set.prototype.add = function (item) {
  this.items.push(item);
};
var obj3 = new $Set();
var obj4 = new $Set();
obj4.add(1);
console.log(obj3.items.length === 1);  // will be true
console.log(obj4.items.length === 1);  // will be true

Solution

// move property initialization into constructor
function Set () {
  this.items = [];
}
Set.prototype.add = function (item) {
  this.items.push(item);
};
var obj5 = new Set();
var obj6 = new Set();
obj6.add(1);
console.log(obj5.items.length === 0);
console.log(obj6.items.length === 1);

References

• coffeescript-class-member