I created a class that supports chaining by making use of return this;, and
now I need to make the current method tell what methods can be chained. Example:
class MyClass {
constructor(path) {
this.path = path;
}
method1() {
// some code here...
// i must make something here to only allow channing if it's method
// blabliblu
return this;
}
blabliblu() {
// some code here...
// same thing here, if the channing is with the method ar_2, it's ok.
return this;
}
ar_2() {
// And so on...
return this;
}
}
So, i can do: method1().blabliblu(), but i can't do method1().ar_2(). Is there lib to help me achieve this?
What you have asked for is not possible in Javascript. When you do:
return this;
in your methods, you are returning the same object. Any public method can be called on the object that is returned and there is no way to specify that because the object was returned from a method it should somehow not allow some methods to be called.
So, using the code in your example where you return this in .method1(), there's no difference between this:
obj.method1();
obj.ar_2();
and this:
obj.method1().ar_2();
That's because the chained version is essentially this internal to the JS interpreter:
let temp = obj.method1();
temp.ar_2();
So, if temp is the same as obj which it is when you return this from method1(), then obj.method1().ar_2(); is just the same as obj.method1(); obj.ar_2(); (with a little less typing). Thus, you can't prevent the calling of .ar_2().
Both just call .method1() and then .ar_2() on the same object. So you can't prevent one scheme, but allow the other. ar_2 is either a public method or it isn't. You can't have it callable in one place and not callable in another on the same object.
Now, you could make obj.method1() return a different object. If you did that, then that different object could have different methods on it and could be an object that does not have a .ar_2() method.
When you chain array methods like this:
let result = [1,2,3].map(...).filter(...);
Each step in that chain is returning a different object (they are not doing a return this, but are creating a new object and returning it. In this specific Array example, these are returning different objects, but of the same type, but you could return different objects of different types. For example:
let result = ["a","b","c"].join("").toUpperCase();
Here, .join() is an Array method, but returns a string object which you can then only call string methods on. So, you could do something like that where you return a different type of object.
Related
I have this chain of methods
MyObject.methodA().methodB().methodC()
Suppose I want methodB() to be added or changed based on some condition:
const inclueMethodB = false;
// change method based on condition
MyObject.methodA().(inclueMethodB ? methodB() : method123()).methodC()
// add method based on condition
MyObject.methodA().(inclueMethodB && methodB()).methodC()
Is there a way to do this in javascript?
I can't think of a way to optionally call a method like that.
If you were choosing between two methods, you could use a subscript where the method name is specified dynamically:
MyObject.methodA()[includeB1 ? "methodB1" : "methodB2"]().methodC()
You could achieve your original goal by defining a method that doesn't do anything except return the same thing that methodB() does. If this is a fluent interface where all the methods just return this, it could be:
doNothing() {
return this;
}
Then you could write:
MyObject.methodA()[includeB ? "methodB" : "doNothing"]().methodC()
But this all seems unnecessarily complex and confusing. Just use ordinary if statements, which express the intent clearly:
let temp = MyObject.methodA();
if (includeB) {
temp = temp.methodB();
}
temp.methodC();
I have a function:
function hello(param){ console.log('param is '+param); }
And two calls. First:
hello(123)
Second:
var a=123; hello(a);
Is there any possible way to tell, from within the hello function, whether param was passed as a var or as a literal value?
NOTICE: I am not trying to solve a problem by this. There are many workarounds of course, I merely wanted to create a nice looking logging function. And also wanted to learn the boundaries of JavaScript. I had this idea, because in JavaScript we have strange and unexpected features, like the ability to obtain function parameter names by calling: function.toString and parsing the text that is returned.
No, primitives like numbers are passed by value in Javascript. The value is copied over for the function, and has no ties to the original.
Edit: How about using an object wrapper to achieve something like this? I'm not sure what you are trying to do exactly.
You could define an array containing objects that you want to keep track of, and check if its in there:
var registry = [] // empty registry
function declareThing(thing){
var arg = { value: thing } // wrap parameter in an object
registry.push(arg) // register object
return arg; //return obj
}
function isRegistered(thingObj){
return (registry.indexOf(thingObj) > -1)
}
var a = declareThing(123);
hello(a);
function hello(param){
console.log(isRegistered(param));
}
An API call I'm making returns empty objects in lieu of null. Tedious doesn't like this, so before I save the API response I'm cleaning the data with the following function:
var object_to_return = input_object;
_.forOwn(object_to_return, function(key_value) {
if (_.isEmpty(key_value)) {
object_to_return[key_value] = null;
}
});
return object_to_return;
This isn't quite correct and I'm curious if anyone knows why and how I can fix it. I'm especially interested in the why and if I should bother with even returning a copy of the object (is it being passed in by reference, or...?)
_.forOwn exposes the key in the callback function; therefore, this worked:
module.exports.convertEmptyObjectsToNull = function(target_object) {
_.forOwn(target_object, function(property, key) {
if (_.isEmpty(property)) {
target_object[key] = null;
}
});
}
Also, as #apsillers mentioned, I wasn't doing much with my assignments, so this method just mutates the input object and doesn't attempt to clone it and return a copy.
In Ruby I think you can call a method that hasn't been defined and yet capture the name of the method called and do processing of this method at runtime.
Can Javascript do the same kind of thing ?
method_missing does not fit well with JavaScript for the same reason it does not exist in Python: in both languages, methods are just attributes that happen to be functions; and objects often have public attributes that are not callable. Contrast with Ruby, where the public interface of an object is 100% methods.
What is needed in JavaScript is a hook to catch access to missing attributes, whether they are methods or not. Python has it: see the __getattr__ special method.
The __noSuchMethod__ proposal by Mozilla introduced yet another inconsistency in a language riddled with them.
The way forward for JavaScript is the Proxy mechanism (also in ECMAscript Harmony), which is closer to the Python protocol for customizing attribute access than to Ruby's method_missing.
The ruby feature that you are explaining is called "method_missing" http://rubylearning.com/satishtalim/ruby_method_missing.htm.
It's a brand new feature that is present only in some browsers like Firefox (in the spider monkey Javascript engine). In SpiderMonkey it's called "__noSuchMethod__" https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/NoSuchMethod
Please read this article from Yehuda Katz http://yehudakatz.com/2008/08/18/method_missing-in-javascript/ for more details about the upcoming implementation.
Not at the moment, no. There is a proposal for ECMAScript Harmony, called proxies, which implements a similar (actually, much more powerful) feature, but ECMAScript Harmony isn't out yet and probably won't be for a couple of years.
You can use the Proxy class.
var myObj = {
someAttr: 'foo'
};
var p = new Proxy(myObj, {
get: function (target, methodOrAttributeName) {
// target is the first argument passed into new Proxy, aka. target is myObj
// First give the target a chance to handle it
if (Object.keys(target).indexOf(methodOrAttributeName) !== -1) {
return target[methodOrAttributeName];
}
// If the target did not have the method/attribute return whatever we want
// Explicitly handle certain cases
if (methodOrAttributeName === 'specialPants') {
return 'trousers';
}
// return our generic method_missing function
return function () {
// Use the special "arguments" object to access a variable number arguments
return 'For show, myObj.someAttr="' + target.someAttr + '" and "'
+ methodOrAttributeName + '" called with: ['
+ Array.prototype.slice.call(arguments).join(',') + ']';
}
}
});
console.log(p.specialPants);
// outputs: trousers
console.log(p.unknownMethod('hi', 'bye', 'ok'));
// outputs:
// For show, myObj.someAttr="foo" and "unknownMethod" called with: [hi,bye,ok]
About
You would use p in place of myObj.
You should be careful with get because it intercepts all attribute requests of p. So, p.specialPants() would result in an error because specialPants returns a string and not a function.
What's really going on with unknownMethod is equivalent to the following:
var unk = p.unkownMethod;
unk('hi', 'bye', 'ok');
This works because functions are objects in javascript.
Bonus
If you know the number of arguments you expect, you can declare them as normal in the returned function.
eg:
...
get: function (target, name) {
return function(expectedArg1, expectedArg2) {
...
I've created a library for javascript that let you use method_missing in javascript: https://github.com/ramadis/unmiss
It uses ES6 Proxies to work. Here is an example using ES6 Class inheritance. However you can also use decorators to achieve the same results.
import { MethodMissingClass } from 'unmiss'
class Example extends MethodMissingClass {
methodMissing(name, ...args) {
console.log(`Method ${name} was called with arguments: ${args.join(' ')}`);
}
}
const instance = new Example;
instance.what('is', 'this');
> Method what was called with arguments: is this
No, there is no metaprogramming capability in javascript directly analogous to ruby's method_missing hook. The interpreter simply raises an Error which the calling code can catch but cannot be detected by the object being accessed. There are some answers here about defining functions at run time, but that's not the same thing. You can do lots of metaprogramming, changing specific instances of objects, defining functions, doing functional things like memoizing and decorators. But there's no dynamic metaprogramming of missing functions as there is in ruby or python.
I came to this question because I was looking for a way to fall through to another object if the method wasn't present on the first object. It's not quite as flexible as what your asking - for instance if a method is missing from both then it will fail.
I was thinking of doing this for a little library I've got that helps configure extjs objects in a way that also makes them more testable. I had seperate calls to actually get hold of the objects for interaction and thought this might be a nice way of sticking those calls together by effectively returning an augmented type
I can think of two ways of doing this:
Prototypes
You can do this using prototypes - as stuff falls through to the prototype if it isn't on the actual object. It seems like this wouldn't work if the set of functions you want drop through to use the this keyword - obviously your object wont know or care about stuff that the other one knows about.
If its all your own code and you aren't using this and constructors ... which is a good idea for lots of reasons then you can do it like this:
var makeHorse = function () {
var neigh = "neigh";
return {
doTheNoise: function () {
return neigh + " is all im saying"
},
setNeigh: function (newNoise) {
neigh = newNoise;
}
}
};
var createSomething = function (fallThrough) {
var constructor = function () {};
constructor.prototype = fallThrough;
var instance = new constructor();
instance.someMethod = function () {
console.log("aaaaa");
};
instance.callTheOther = function () {
var theNoise = instance.doTheNoise();
console.log(theNoise);
};
return instance;
};
var firstHorse = makeHorse();
var secondHorse = makeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
This doesn't work for my use case as the extjs guys have not only mistakenly used 'this' they've also built a whole crazy classical inheritance type system on the principal of using prototypes and 'this'.
This is actually the first time I've used prototypes/constructors and I was slightly baffled that you can't just set the prototype - you also have to use a constructor. There is a magic field in objects (at least in firefox) call __proto which is basically the real prototype. it seems the actual prototype field is only used at construction time... how confusing!
Copying methods
This method is probably more expensive but seems more elegant to me and will also work on code that is using this (eg so you can use it to wrap library objects). It will also work on stuff written using the functional/closure style aswell - I've just illustrated it with this/constructors to show it works with stuff like that.
Here's the mods:
//this is now a constructor
var MakeHorse = function () {
this.neigh = "neigh";
};
MakeHorse.prototype.doTheNoise = function () {
return this.neigh + " is all im saying"
};
MakeHorse.prototype.setNeigh = function (newNoise) {
this.neigh = newNoise;
};
var createSomething = function (fallThrough) {
var instance = {
someMethod : function () {
console.log("aaaaa");
},
callTheOther : function () {
//note this has had to change to directly call the fallThrough object
var theNoise = fallThrough.doTheNoise();
console.log(theNoise);
}
};
//copy stuff over but not if it already exists
for (var propertyName in fallThrough)
if (!instance.hasOwnProperty(propertyName))
instance[propertyName] = fallThrough[propertyName];
return instance;
};
var firstHorse = new MakeHorse();
var secondHorse = new MakeHorse();
secondHorse.setNeigh("mooo");
var firstWrapper = createSomething(firstHorse);
var secondWrapper = createSomething(secondHorse);
var nothingWrapper = createSomething();
firstWrapper.someMethod();
firstWrapper.callTheOther();
console.log(firstWrapper.doTheNoise());
secondWrapper.someMethod();
secondWrapper.callTheOther();
console.log(secondWrapper.doTheNoise());
nothingWrapper.someMethod();
//this call fails as we dont have this method on the fall through object (which is undefined)
console.log(nothingWrapper.doTheNoise());
I was actually anticipating having to use bind in there somewhere but it appears not to be necessary.
Not to my knowledge, but you can simulate it by initializing the function to null at first and then replacing the implementation later.
var foo = null;
var bar = function() { alert(foo()); } // Appear to use foo before definition
// ...
foo = function() { return "ABC"; } /* Define the function */
bar(); /* Alert box pops up with "ABC" */
This trick is similar to a C# trick for implementing recursive lambdas, as described here.
The only downside is that if you do use foo before it's defined, you'll get an error for trying to call null as though it were a function, rather than a more descriptive error message. But you would expect to get some error message for using a function before it's defined.
I have a function that takes a string object name and I need the function to create an new instance of a object that has the same name as the value of the string
For example,
function Foo(){}
function create(name){
return new name();
}
create('Foo'); //should be equivalent to new Foo();
While I know this would be possible via eval, it would be good to try and avoid using it. I am also interested if anyone has an alternative ideas to the problem (below)
I have a database and a set of (using classical OO methodology) classes, roughly one for each table that define common operations on that table. (Very similar to Zend_Db for those who use PHP). As everything is asynchronous doing tasks based on the result of the last one can lead to very indented code
var table1 = new Table1Db();
table1.doFoo({
success:function(){
var table2 = new Table2Db();
table2.doBar({
notFound:function(){
doStuff();
}
});
}
});
The obvious solution is to create helper methods that abstracts the asynchronous nature of the code.
Db.using(db) //the database object
.require('Table1', 'doFoo', 'success') //table name, function, excpected callback
.require('Table2', 'doBar', 'notFound')
.then(doStuff);
Which simplifies things. However the problem is that I need to be able to create the table classes, the names of which can be inferred from the first augment passed to require which leads me to the problem above...
Why not simply pass the constructor function into the require method? That way you sidestep the whole issue of converting from name to function. Your example would then look like:
Db.using(db) //the database object
.require(Table1Db, 'doFoo', 'success') //table constructor, function name, expected callback
.require(Table2Db, 'doBar', 'notFound')
.then(doStuff);
However, if you really want to use a string...
Why are you deadset on avoiding using eval? It is a tool in the language and every tool has its purpose (just as every tool can be misused). If you're concerned about allowing arbitrary execution, a simple regular expression test should render your usage safe.
If you're dead-set on avoiding eval and if all of your constructor functions are created in the default global scope (i.e. the window object), this would work:
function create(name) {
return new window[name]();
}
If you want to get fancy and support namespace objects (i.e. create('MyCompany.MyLibrary.MyObject'), you could do something like this:
function create(name) {
var current,
parts,
constructorName;
parts = name.split('.');
constructorName = parts[parts.length - 1];
current = window;
for (var i = 0; i < parts.length - 1; i++) {
current = current[parts[i]];
}
return new current[constructorName]();
}
You were at the gate of completeness. While Annabelle's solution let's you to do what's you've just wanted in the way you wanted (passing strings), let me offer you an alternative. (passing function references)
function Foo(){}
function create(name){
return new name();
}
create(Foo); // IS equivalent to new Foo();
And voila, it works :) I told you. You were at the doorsteps of the solution.
What happened is that you've try to do this
new 'Foo'()
Which doesn't makes much sense, does it? But now you pass the function by reference so the line return new name(); will be transformed into return new Foo(); just how you would expect.
And now the doors are opened to abstract the asynchronousness of your application. Have fun!
Appendix: Functions are first-class objects, which means that they can be stored by reference, passed as an argument by reference or returned by another function as values.