I want to decorate a function of a JavaScript "class" (prototype):
SomeClass = function() { this.someVar = 5; };
SomeClass.prototype = {
foo: function(arg) { /* do something with this.someVar */ }
}
However, I cannot change the source of SomeClass nor am I able to influence the instance creation of SomeClass instances.
Therefore I thought about doing the following:
var oldFoo = SomeClass.prototype.foo;
SomeClass.prototype.foo = function(arg) {
console.log("Yey, decorating!");
oldFoo(arg);
};
This seems to work fine, however, due to function scoping oldFoo cannot access someVar anymore (the this object is now window). How to overcome this issue?
You need to delegate it correctly. What's happening is that since you're calling oldFoo like a bare function the this value is set to undefined (or the global object in non-strict mode).
Apply the method with the arguments and set the this value explicitly:
oldFoo.apply(this, arguments); // use the same `this` and same arguments as called.
Note that in order to be really correct - you also need to return the result. So in total your code should look something like:
SomeClass.prototype.foo = function(arg) {
console.log("Yey, decorating!");
return oldFoo.apply(this, arguments); // alternatively capture the return value
}; // and process it and then return it
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