I've seen something similar to this code in the Google API JavaScript, I mean the r=Array part. Here is an example of what they have done:
var r = Array;
var t = new r('sdsd' , 'sdsd');
alert(t[0]);
Few questions about this:
Is it legal to write like this and won't cause any problems?
I can do something similar with other keywords like ´For´ loop or with the ´this´ keyword?
Can I have article about this JavaScript official keyword shortcuts etc..?
Thank you in advance.
That works because Array is an object. You can do that with any object. For example, the Date object:
var d = Date;
console.log((new d()).getTime()); //Prints time
You cannot do that for keywords such as for or while because they are language constructs that will be recognised by the interpreter.
You can do it with this:
document.getElementById("b").onclick = function() {
var x = this; //this holds a reference to the DOM element that was clicked
x.value = "Clicked!";
}
In fact, that can be very useful sometimes (to keep a reference to this so you can access it from an anonymous inner function for example). This also works because, to put it simply, this will be a reference to an object.
Yes
for - no. this - yes.
You can store references to any JavaScript object in a variable. String, Array, Object, etc. are JavaScript objects that are built-in to the language. for, if, while, etc. are are JavaScript statements, and cannot be stored or referenced any other way.
You can do it the other way around as well (and really mess yourself up in the process):
Array = 0;
var myArray = new Array("a", "b", "c"); // throws error
This is easily undone like this:
Array = [].constructor;
Edit: Being able to assign the value of this to a variable is essential when nesting functions that will execute in a different scope:
function Widget() {
var that = this;
this.IsThis = function() {
return isThis();
};
function isThis() {
return that == this;
}
}
new Widget().IsThis(); // false!
Maybe not the best example, but illustrates losing scope.
You cannot reassign the value of this:
function doSomething() {
this = 0; // throws error
}
Related
I have a code that is supposed to read a google sheet and push these into an array but i keep encountering an error that proData.push is not a function What could I be missing in this code?
function getData() {
var values = SpreadsheetApp.getActive().getSheetByName("Projects").getRange("A1:O").getValues();
values.shift();
var proData = [];
values.forEach(function(value) {
var proData = {};
proData.project_state = value[0];
proData.project_name= value[4];
proData.project_code= value[5];
proData.end_date= value[2];
proData.push(proData);
})
Logger.log(JSON.stringify(proData));
return proData;
}
I will appreciate help in looking at this.
This line tells the whole story:
proData.push(proData);
You're trying to push an object onto itself? Clearly this is an indication that something is wrong. So let's look at where you define proData:
var proData = {};
That at least explains the error. proData is an object, not an array. And an object indeed doesn't have a function called push. You may have thought it was an array, because you defined an identically named array here:
var proData = [];
But for the line where you call .push, how is the system to know which variable you intend for what purpose? In a higher scope you have an array named proData, but in the current scope of this operation you obscured that with an object named proData. And in doing so made the array inaccessible within the scope of the function passed to .forEach.
To avoid confusing both the JavaScript engine and yourself in this matter, simply use different variable names. Re-naming the variable in the smaller scope has a lower impact, so that's a good candidate. (Though it's not always the best choice. If the variable in the higher scope is semantically not clear about what it contains then it should be re-named.)
Something like this:
values.forEach(function(value) {
var pd = {};
pd.project_state = value[0];
pd.project_name = value[4];
pd.project_code = value[5];
pd.end_date = value[2];
proData.push(pd);
});
Is it possible to keep an object reference without using an holder object in javascript?
Currently when an object gets overridden I sometimes lose the reference to the "current" object state illustrated in the snippet below;
Is there a way to put a "pointer" in an array or not?
EDIT
To the questions asked:
What I have in the objects I have are references to form fields. Some of these are text fields, some of them are textareas, some of them checkboxes.
I wish to keep a map next to the direct referene of what type they are.
basicaly it would be
obj {
this.text1 = createTextField();
this.text1.datepicker();
this.text2 = createTextField();
this.area1 = createArea();
this.check = createCheck();
this.datefields = [this.text1];
this.checkboxes = [this.check];
}
So I can use the datefields/checkboxes array as a checkpoint to validate against which type a field is/should behave.
Currently I use
function datefields() { return [this.text1]; };
But I'd like to know if there's a better way to do this than to intantiate a new array when I need to check it.
I know there is a way with observers to mimic pointer behaviour, and i've fiddled with those and have some good results with that, i'm just curious if there are other ways i'm not aware of.
function myObject() {
this.myvalue = null;
this.arr = [this.myvalue];
}
myObject.prototype.alter = function() {
this.myvalue = "hello";
}
var x = new myObject();
var elem = document.getElementById('results');
function log(message) {
elem.appendChild(document.createTextNode(message));
elem.appendChild(document.createElement('br'));
}
log("x.myvalue = "+x.myvalue);
log("x.arr[0] = "+x.arr[0]);
log("calling alter");
x.alter();
log("x.myvalue = "+x.myvalue);
log("x.arr[0] = "+x.arr[0]);
<div id="results"></div>
Simple answer: Only objects (including all subtypes) are passed by reference in JS. All other simple values are copied.
For a bit more detail I would recommend reading You Don't Know JS: Types & Grammer but specifically the section Value vs Reference in Chapter 2:
In JavaScript, there are no pointers, and references work a bit differently. You cannot have a reference from one JS variable to another variable. That's just not possible.
Quoting further on:
Simple values (aka scalar primitives) are always assigned/passed by value-copy: null, undefined, string, number, boolean, and ES6's symbol.
Compound values -- objects (including arrays, and all boxed object wrappers -- see Chapter 3) and functions -- always create a copy of the reference on assignment or passing.
There are plenty of examples included to show these points. I would highly recommend reading through to get a better understanding of how values/references work in JS.
There is no pointers in Javascript, though you could cheat a little using a wrapper object. Here is a minimal implementation of such an object:
var Wrapper = function (value) {
this.value = value;
};
Wrapper.prototype.valueOf = function () {
return this.value;
};
Then you may use it in place of the original value:
function myObject() {
this.myvalue = new Wrapper(null); // wrapper
this.arr = [this.myvalue];
}
myObject.prototype.alter = function() {
this.myvalue.value = "hello"; // notice the ".value"
}
The rest of your code needs no tweaks.
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.
This question just got upvoted so can update question with what I did
I solved it by iterating over the window object (or user specified object root) and when I found the correct instance I backtracked and got the name from the index. The final solution can be found here
https://github.com/AndersMalmgren/Knockout.BindingConventions
Update end
I'm planning on writing a convention over configuration template source engine for KnockoutJS / MVC.
I'm started with a little client side POC and ran into a show stopper right away
My plan is use this syntax or something similar
MyApp.EditCustomersViewModel = function() {
ko.templates.loadView(this);
};
When doing this it will check the tamplate cache or fetch the templates from server using the object name as key.
The problem is I cant get the name of the prototype object, i tried this
Object.prototype.getName = function() {
var funcNameRegex = /function (.{1,})\(/;
var results = (funcNameRegex).exec((this).constructor.toString());
return (results && results.length > 1) ? results[1] : "";
};
If works for objects defined like this
function MyClass() {
}
If you add a prototype to the above object it will not work, or if you define it like this
MyApp = {};
MyApp.MyClass = function() {
};
Prototype and scoping is two musts so this is a showstopper, any ideas?
Fiddle: http://jsfiddle.net/aRWLA/
edit: The background for this is like this.
On the server you have structure like this
Templates\ [ViewName]\index.html
Templates\ [ViewName]\sub-model-template.html
on the client you will do
MyApp.EditCustomersViewModel = function() {
ko.templates.loadView(this);
};
which will generate a ajax request with the objects name as key, which will fetch all the templates for the view in question
Only hoisted functions (function someFunc() {) have a retrievable name.
Assigned functions do not, because you are not technically naming the function but creating an anonymous function and assigning a reference to it (in the memory) to a named variable.
So it's the var, not the function, that is named.
This makes the very idea of retrieving function names pretty much a none-starter, since in any vaguely mature pattern you'll be writing methods, not hoisted functions - and methods of course are assigned functions.
Named expressions (see other answers) are a partial workaround but these have other issues - not least lack of support in older IEs.
(Sidenote: I've long expected browser vendors to build around this such that the names of assigned functions became retrievable, but no joy yet AFAIK.)
I think you problem in improper replacing function prototype: if you replace function prototype object then you must preserve constructor member in prototype:
function Test1() {
}
Test1.prototype={
constructor: Test1
};
MyApp={};
MyApp.MyClass=function MyClass(){
};
MyApp.MyClass.prototype={
constructor: MyApp.MyClass
};
Your example: http://jsfiddle.net/aRWLA/1/
Modified example: http://jsfiddle.net/aRWLA/2/
You can make use of named function expressions:
MyApp.MyClass = function MyClass() { ... };
But note that (suprise) they don't work correctly in all versions of IE.
See: http://kangax.github.com/nfe/
THIS DOES NOT ANSWER THE QUESTION
However, the code might be useful to other people, so I'm leaving it here, just in case. I don't expect upvotes, but please don't abuse it for downvoting either. Thanks.
I don't know your use case, as such I think you've got a design issue - the problem you describe shouldn't happen in practice.
But let's say you do need to have this working. An easy way to do what you need would be something like:
function findNamed(obj, fn){
for(var p in obj)
if(obj[p] === fn)
return p;
return false;
}
var m = {};
m.MyClass = function() {};
console.log(findNamed(m, m.MyClass));
Of course, the solution could be made into a more appropriate OOP form, but this is just to give an idea.
To replicate your use case, it would look like:
m.MyClass = function() {
findNamed(this, arguments.callee);
};
So, the final code is:
Object.prototype.getNameOfCall = function(fn) {
for(var p in this)
if(this[p] === fn)
return p;
throw "Callback not in object.";
};
var m = {};
m.MyClass = function() {
console.log(this.getNameOfCall(arguments.callee)); // MyClass
};
m.MyClass(); // test it out
I had the following question in a test today. But i had not see something like functionName.VariableName before. Not sure how that works.
Would be great if you can tell me the solution:
function Item(itemName)
{
var next_item_id = 1;
Item.item_name = itemName;
Item.item_id = next_item_id++;
}
var Item1 = Item('Desktop');
var Item2 = Item('Laptop');
var Item3 = Item('Monitor');
Anything wrong with the code above? if yes fix it. (The problem i would see is next_item_id is always 1, need to make it global?)
Modify the function so that the variable “next_item_id”, cannot be modified during run time.
My own question, how does the variable like Item.item_name work? I want to google it, but not sure what I should search for.
Thanks.
Your thinking is close in that next_item_id will always be 1, but it's generally not recommended to pollute the global namespace. Instead, wrap it in an anonymous function:
(function() {
var next_item_id = 1;
function Item(itemName)
{
//Use "this" to apply the property to the instance only
this.item_name = itemName;
this.item_id = next_item_id++;
}
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
})()
Also, as in Java, the general best practice in Javascript is to use camelCase rather than under_scores. Constructors are generally in UpperCamelCase. Examples:
Array //UpperCamelCase
Object.prototype.toString //toString is camelCase
This question is ambiguous, and, depending on the interpretation, there can be a number of possible answers. next_item_id is a "var" declared inside a function and naturally has an internal [[DontDelete]] and [[DontEnum]] flag. If it's a "var" and we're not using "this" as shown in my modified code, the variable is inherently not accessible outside of the function or its nested functions and therefore cannot be modified. You can use the non-standard const or you can create an object and use Object.defineProperty to create a setter that returns false assuming an ES5-compatible environment, etc.
Functions are objects in Javascript. All objects can have "properties."
Javascript functions are also objects and can have properties.
They behave like static fields in Java.
You're going about it all wrong. I'm assuming that you want to set internal variables.
If you want to create a 'constructor', do something like this:
function Item(itemName)
{
var next_item_id = 1;
this.item_name = itemName;
this.item_id = next_item_id++;
}
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
Both item_name and item_id will be publicly available. To make use next_item_id like you want, try this:
Item.next_item_id = 1;
Then in your constructor, do Item.next_item_id++;
Your final code should look something like this:
function Item(itemName)
{
this.item_name = itemName;
this.item_id = Item.next_item_id++;
}
Item.next_item_id = 1;
var Item1 = new Item('Desktop');
var Item2 = new Item('Laptop');
var Item3 = new Item('Monitor');
What this is doing is attaching a property to the object Item. You might be thinking, "Item is a function! How is this possible?!". Nearly everything in JavaScript is an object with mutable properties. The only things that aren't are the keywords null and undefined (AFAIK).
Item is still a function, but it also has a property next_item_id.
Also, using new will create a new instance. This is a similar concept as in Java and other programming languages. Just calling Item is like calling a function, and you'll get back whatever is returned from it (through an explicit return statement, otherwise undefined).
Answer for question 2 : Modify the function so that the variable “next_item_id”, cannot be modified during run time.
If I have interpreted your question correctly, you want to have Item.item_id as a constant that cannot be modified any where outside the constructor. I don't think we can have constants in JS.
Item1.item_id = someValue
Above line, used some where can change the item_id value of Item1.