Javascript, getting away from 'this' and using local scope - javascript

In a typical js class, all calls to member functions must be preceded by this. I was looking at a technique that would let me create a library of inter-dependent STATIC functions and relying on closure/scope to make things a bit easier.
Example:
var Singleton={
//main entry point
// call with fn name, args...
call:function(){
var args=[];
if (arguments.length==0) {
return;
}
// get the fn name
var fn=arguments[0];
var x;
// make args array
for (x=1;x<arguments.length;x++) {
args[args.length]=arguments[x];
}
// I want to get rid of this part
// See below for what I wish
// Here I have access to fns below due to hoisting in js
// so I put them in a map...
var fns={
test:test
// etc, more like this I do not want to type/maintain
}
// ... all so I can do this
// get my function.
var fun=fns[fn];
// instead of that, I would like to "override whitespace" and
// say something like:
// var fun=['fn_name'];
// so I can index into local scope and get a fn
//
// log error if not found
if (typeof fun=='undefined') {
loge('Singleton: function not found:'+fn);
return;
}
// ok, run the function
return fun.apply(window,args);
// the test fn accesses test2() without dot notation
function test(a){
// Note: here in test fn it can access test2()
// without using this.test2() syntax
// as you would in normal objects
var s=test2();
alert(s+' test:'+a);
};
function test2(){
return 'test2';
};
}
}
I was hoping someone more familiar with advances in javascript might have advice on how to emulate an "implied but unnecessary this", it always struck me as strange that this defaults to window, and wouldn't it be nice if this could be pointed to an anonymous object with the local scope attached.
I would love to say ['localObject'] to get something in scope.
Edit:
After seeing some of the responses, I will restate this in the form of a challenge:
What I am looking for is a syntax cheat, a way to, as #Varuna put it, "1. Access static methods without using this variable i.e. they will remain global to one another. 2. Do not want to maintain a local array for static methods and want to achieve with in the local scope itself."
Put differently, I need to have the declared functions Register themselves, but I don't want to state the function name more than once. I suppose #Varuna has a solution using eval to access the local scope.
The following approach wont work:
var o={};
o['fn']=function fn(){};
o['fn2']=function fn2(){};
...because you have to state the fn name twice, but closure is preserved.
And this:
var a=[
function fn(){}
,function fn2(){}
];
Register(a);
Will not work because, AFAIK, you lose closure, ie. fn2 cannot see fn. Which also makes the following declarative style a "this nightmare":
window.MINE={
fn:function fn(){
//this?
// want to say fn2(), not this.fn2(), nor MINE.fn2()
}
,fn2:function fn2(){
//this?
}
,deeper:{
//more
}
};
But something like this might work, if you created a weird property that does the registration on assignment:
var registar=new Registar();
registar.reg=function fn(){};
registar.reg=function fn2(){};
//then access
var fn=registar.getFn(n);
// or
var fn=registar._[n];
The above relies on js properties and having access to fn.name, which is not available in all cases AFAIK.

If I understand correctly, you want to create objects that:
have static members
... which can be accessed without using the this notation
The easiest solution (assuming I've properly understood your query), would be to simply use a closure to store your stratic fields, access them directly by name, then explicitly add them as object members.
Consider:
var myConstructor = (function(){
var foo = 'someStaticField';
var bar = function(){
alert('A static method returns ' + foo);
};
return function(){
return {
foo : foo,
bar : bar
};
};
})();
var myInstance = new myConstructor();

As per my understanding, you want to:
1. Access static methods without using this variable i.e. they will remain global to one another.
2. Do not want to maintain a local array for static methods and want to achieve with in the local scope itself.
You can check whether a method exist using eval.Check Here
Only drawback is that this will be using eval method.
Code will be:
var Singleton = {
//main entry point
// call with fn name, args...
call: function () {
var args = [];
if (arguments.length == 0) {
return;
}
// get the fn name
var fn = arguments[0];
var x;
// make args array
for (x = 1; x < arguments.length; x++) {
args[args.length] = arguments[x];
}
//check whether function exist in local scope and not in global scope
if (typeof eval(fn) !== 'undefined' && typeof window[fn] === 'undefined') {
// ok, run the function
return eval(fn).apply(window, args);
}
else{
// log error if not found
loge('Singleton: function not found:' + fn);
return;
}
// the test fn accesses test2() without dot notation
function test(a) {
// Note: here in test fn it can access test2()
// without using this.test2() syntax
// as you would in normal objects
var s = test2();
alert(s + ' test:' + a);
};
function test2() {
return 'test2';
};
}
}

How about declaring functions that can access each other in separate closure, and exporting them to main method by binding your call method to an object containing the functions? Something like previous post (modified slightly):
var Singleton = {
call: (function() {
// here 'call' is bound to object containig your test functions
// this: {test, test2}
if (0 == arguments.length) return;
// log error if not found
if ('function' != typeof this[arguments[0]]) {
console.warn('Singleton: function not found:' + arguments[0]);
return;
}
// '...index into local scope and get function
// ie. get the function by it's name
return this[arguments[0]].
apply(window, Array.prototype.slice.call(arguments, 1));
// --- or:
// you can explicitly introduce function names to current scope,
// by `eval`-ing them here (not very much preferred way in JavaScript world):
for (var fname in this)
if (this.hasOwnProperty(fname))
eval('var ' + fname + ' = ' + this[fname]);
// and you can reference them directly by using their names
var fn = eval(arguments[0]);
return fn.apply(window, Array.prototype.slice.call(arguments, 1));
}).bind(
(function() {
var _exports = {};
function test (a) {
var s = test2();
alert(s + ' test: ' + a);
}
function test2 () {
return 'test2';
}
_exports['test'] = test;
_exports['test2'] = test2;
return _exports;
})()
)};
Singleton.call('test', 'foo and stuff');
//
previous post:
You are talking about Function#bind functionality that enables 'customizing' function's context. .bind() your call method to required 'local context' like this:
var Singleton = {
//main entry point
// call with fn name, args...
call: (function() {
// here `this` (context) is object bound to `call` method
// not `global` object, which is default for 'unbound' functions
var locals = this; // {fns, shift, loge, isfunc}
var fn;
var fun;
var x;
if (arguments.length == 0)
return;
// get the fn name
fn = locals.shift(arguments);
// '...index into local scope and get a fn'
fun = locals.fns[fn];
// log error if not found
if (!locals.isfunc(fun)) {
locals.loge('Singleton: function not found:' + fn);
return;
}
// ok, run the function
return fun.apply(window, arguments);
// lock `call`'s context to provided object
// and use `this` to reference it inside `call`
}).bind({
fns: (function(_) {
// and you can '...create a library of inter-dependent STATIC functions'
// in this closure and invoke them in `call` method above
_.test = function (a) {
var s = _.test2();
alert(s + ' test: ' + a);
};
_.test2 = function() {
return 'test2';
};
return _;
})({}),
// and create couple of helper methods as well...
isfunc: (function(_getclass) {
_getclass.func = _getclass(_getclass);
return ('function' !== typeof(/foo/)) ?
function(node) {
return 'function' == typeof node;
} :
function(node) {
return _getclass.func === _getclass(node);
};
})(Function.prototype.call.bind(Object.prototype.toString)),
loge: console.warn,
shift: Function.prototype.call.bind(Array.prototype.shift)
}),
};
Singleton.call('test', 'foo and stuff');
// eof

Here's one 'in your face answer', because I really don't like what I see here.
I don't see why you need this kind of construct, you already have that as part of language core.
1. dynamic lookup
you are doing it in a rather 'unprecedented' kind of way,
hashes already do that for you, and it's lightning fast to do a hash search.
If you are eval()-ing random strings to do simple name lookup you really have to
step aside from a keybord for a while... (no offense please)
2. closures
you are saying about 'using closures' which you actualy don't use.
your call function redeclares test functions each time it gets called,
and looks the ('fresh version') functions in it's own variable scope table,
instead of lookig them up in parent scope chains (aka. closures)
outside it's lexical location
3. nfe vs. nfd
ie. named function expressions vs. named function declarations
...you cannot assign a function to a local var and have it retain closure.
It is a feature, you might not be aware of how it works (it tripped me up as well).
check this article out for clarification
4. exceptions
Singleton: function name not found... x4!
Just go ahead and call a function,
interpreter will throw for you anyway if it cannot find/execute
5. eval (aka. ^^)
Singleton.call.ctx.fun = eval(Singleton.call.ctx.fn);
eval takes any string here(#!), and gladly executes ones like:
'for(;;);', or 'while(1);'... forever.
You probably don't want to have any code running unless it was your stuff.
6. arguments handling
It is considered best practice out there to use single (Object) options parameter
to 'fine tune' any significant piece of bundled functionality,
instead of trying to figure that out by type checking provided argument list
Here's, in couple of simple lines, what I (and as I can see #Jimmy Breck-McKye) suggest you should do:
var Singleton.call = (function () {
var funcmap = {
'f_1': function () {},
// etc.
'f_N': function () {},
};
return function (options) {
// options members:
// context, (Object) context, (defaults to global if none is given)
// func, (String) function_name,
// args, (Array) arguments to pass into a function
// this line does everything your 100+ lines long snippet was trying to:
// look's up parent scope for a function, tries to run it
// passing provided data, throws if it gets stuck.
return funcmap[options.func].apply(options.context, options.args);
};
})();
//

Answering my own question here.
The core of the issue is that you cannot assign a function to a local var and have it retain closure.
Consider that when writing a function with global and window scope, this is not necessary to call another function with identical scope. Such is not the case with member functions.
Another way of saying this is that there is no space where your cursor can sit and as you declare a function it automatically gets attached to the current this.
function fn(){}// if we are in global scope, then window.fn becomes defined
// but if we are inside, say, a constructor, simple declaration will not attach
// it to this, but fn is available in scope.
Any assignment on function declaration BREAKS part of the expected closure:
var IdentifierAvailableToClosure=function Unavailable(){}
But assignment after declaration works:
function NowAvailable(){}
var SynonymAvailableToo=NowAvailable;
This is what I meant by not wanting to repeat the name twice to get the mechanism to work.
This fact made me abandon other methods and rely on eval as suggested. Here is a first draft:
// This object is an encapsulation mechanism for a group of
// inter-dependent, static-ish, functions that can call each other
// without a this pointer prefix.
// Calls take the form of:
// Singleton.call(functionName:String [,arg1]...)
// or
// Singleton.call(contextObject:Object, functionName:String [,arg1]...)
// If a context is not provided, window is used.
//
// This type of mechanism is useful when you have defined a group
// of functions in the window/global scope and they are not ready
// to be formalized into a set of classes, or you have no intention
// of doing that
//
// To illustrate the issue, consider that a function
// which is defined in window/global scope
// does not have to use the this pointer to call a function of
// identical scope -- yet in a class member function, the this pointer
// MUST be used
// Therefore, trying to package such functions requires injecting
// the this pointer into function bodies where calls to associater
// functions are made
//
// Usage is primarily for development where one has control over
// global namespace pollution and the mechanism is useful in
// refactoring prior to formalization of methods into classes
var Singleton={
// Main call point
call:function(){
// Bail with error if no args
if (arguments.length==0) {
throw('Singleton: need at least 1 arg');
}
// As all functions in the local scope library below
// have access to the local scope via closure, we want to reduce
// pollution here, so lets attach locals to this call
// function instead of declaring locals
//
// Prepare to call anon fn
Singleton.call.args=arguments;
// Make ctx have args, context object, and function name
Singleton.call.ctx=(function (){// return args,ctx,name
// out
var args=[];
//locals
var x, fn;
// collapse identifier
var a=Singleton.call.args;
// closure object avail to functions, default to window
that=window;
// first real function argument
var arg_start=1;
// first arg must be function name or object
if (typeof a[0]=='string') {// use window ctx
fn=a[0];
// if first arg is object, second is name
}else if (typeof a[0]=='object') {
// assign given context
that=a[0];
// check second arg for string, function name
if (typeof a[1]!='string') {
var err='Singleton: second argument needs to be a fn name'
+' when first arg is a context object';
throw(err)
return;
}
// ok, have a name
fn=a[1];
// args follow
arg_start=2;
}else{
// improper arg types
var err='Singleton: first argument needs to be a string or object';
throw(err)
}
// build args array for function
for (x=arg_start;x<a.length;x++) {
args[args.length]=a[x];
}
// return context
return {
args: args
,that:that
,fn:fn
};
})();
// using function library present in local scope, try to find specified function
try{
Singleton.call.ctx.fun=eval(Singleton.call.ctx.fn);
}catch (e){
console.error('Singleton: function name not found:' + Singleton.call.ctx.fn);
throw('Singleton: function name not found:' + Singleton.call.ctx.fn);
}
// it must be a function
if (typeof Singleton.call.ctx.fun !== 'function') {
console.error('Singleton: function name not found:' + Singleton.call.ctx.fn);
throw('Singleton: function name not found:' + Singleton.call.ctx.fn);
}
// library functions use that instead of this
// that is visible to them due to closure
var that=Singleton.call.ctx.that;
// Do the call!
return Singleton.call.ctx.fun.apply(that, Singleton.call.ctx.args);
//
// cool library of functions below,
// functions see each other through closure and not through this.fn
function test(s){
alert(test2()+' test:'+s);
}
function info_props(){
console.info(this_props());
}
function test2(){
return 'test2';
}
function this_props(){
var s='';
for (var i in that) {
s+=' '+i;
}
return s;
};
}
};

Related

Call function on object literal, represented by string - JavaScript

I am using history.js. In the stateObj of the pushState function, I want to add a reference to a function (Car.init(), or Boat.init(). In C++, I believe I could use a function pointer.
Then on window.onpopstate, I want to reference that function and call it. I can read the string (Car.init(), but then how can I call the function? I don't want to use eval.
You probably shouldn't, but if you do want to invoke a function based on a global dotted-path name, that could be accomplished like this:
function callFunction(name, var_args) {
// break the string into individual property/method names
var parts = name.split('.');
// start with a reference to the global object
var target = window;
var previousTarget = null;
for (var i = 0; i < parts.length; i++) {
// keep a copy of the previous reference to use as the `this` value
previousTarget = target;
// change the reference to the next named property
target = target[parts[i]];
}
// grab the remaining arguments
var args = Array.prototype.slice.call(arguments, 1);
// call the target function, with previousTarget as the subject, using args
return target.apply(previousTarget, args);
}
// This is in the top-level/global scope. This won't work for a local definition.
var MyApp = {
currentUser: {
name: 'Joe',
displayName: function(greeting) {
alert(greeting + " ," + this.name + "!");
}
},
openBar: function() {
alert("The Foo Bar is now open for business!");
}
};
var functionName = 'MyApp.currentUser.displayName';
callFunction(functionName, "Hello");
This is safer than using eval (good call on avoiding it), but is still pretty wacky and very difficult for JavaScript interpreters to optimize. Instead, the recommended approach is to use a reference (pointer) to the function. This is probably similar to what you'd do in C++. If the function doesn't use this (i.e. if it's a static function, not a method), you can just take a reference to the function directly.
var open = MyApp.openBar;
open();
If it does have a this value, you'll need to use the .bind() method to preserve its association with the object it's attached to.
var display = MyApp.currentUser.displayName.bind(MyApp.currentUser);
display("Greetings");
If you pass additional arguments to .bind(), you can also specify the leading arguments that will be used to call the function.
var displayHello = MyApp.currentUser.displayName.bind(MyApp.currentUser, "Hello");
displayHello();

javascript redefine prototype using existing prototype

I would like to redefine a prototype, but still use the original prototype function. The purpose is to add test bench debug code to an existing production prototype definition.
// production definition
function tester(n) { this.name = n ; }
tester.prototype.getName = function() { return this.name; }
// debug override
if (typeof tester.prototype.getName === 'function') {
var f = tester.prototype.getName;
tester.prototype.getName = function () {
return f() + " (plus some more)";
};
}
// test
var tt = new tester('testing');
console.log(tt.getName()); // logs ' (plus some more)' but I want 'testing (plus some more)'
Naturally, my production prototypes are much more complex, and I would rather not embed my test bench code in the production code.
Many thanks in advance! - John :)
You have to call f with the correct this value and the correct arguments:
f.apply(this, arguments);
Learn more about this and apply.
This seems to be a very fragile approach though because it also requires you to know what kind of value the original method returns and what calling code is doing with it.
But I guess you are only doing this for very specific methods.
In your redefinition, you need to set the proper context (this) for the f function using Function.prototype.apply(). In your code, the context of the f function is set to the global scope, even when it is called in the redefined getName property of tt. Instead, it should be the context of the redefined getName function.
// production definition
function tester(n) { this.name = n ; }
tester.prototype.getName = function() { return this.name; }
// debug override
if (typeof tester.prototype.getName === 'function') {
var f = tester.prototype.getName;
// Here, 'this' is the global scope (window).
console.log('Global scope:');
console.log(this);
console.log('f called with context of global scope:');
console.log(f()); // prints 'name' prop of global scope
tester.prototype.getName = function () {
console.log("Instance:");
console.log(this);
// Here, 'this' is tt when tt.getName() is evaluated.
// If we call f() here, the context for f
// will still be the global scope, instead of tt.
// So, we use apply to set the context of f to
// an instance of tester.
return f.apply(this) + " (plus some more)";
};
}
// test
var tt = new tester('testing');
console.log(tt.getName()); // logs testing (plus some more)'
http://jsfiddle.net/a27xzqc9/3/
Extending Felix and Patrick's answer with encapsulation to avoid using the global variable "f", since I will be reusing this structure several times.
;(function(){
if (typeof tester.prototype.getName === 'function') {
var f = tester.prototype.getName;
tester.prototype.getName = function () {
return f.apply(this) + " (plus some more)";
};
}
})();

Determine function has defined as a expression or declaried as function

I want to know how the function has been initialized, with the expression or declaried as fuction. _ Amazon interview question
expression : var a = function (){ }
declaration: function a (){ }
You could just do a.toString() and parse out the name. Or do the same with regular expressions
a.toString().test(/^\s*function\s*\(/);
function a(){ }; // gives false
var a = function (){ }; // gives true
Of course as Grundy pointed out this fails with named functions. Something like
var a = function b() {};
or
function b() {};
var a = b;
And ES6 has .name (see the Browser table at the bottom for the current state of affairs) - https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Function/name
I don't think it's possible to do so. The only difference between:
var func = function(){ };
and:
function func() { };
Is that the first one gets assigned on runtime. The way I see it, is that both function statements return a reference to their respective function objects. In that sense they are both the same. The only thing you could argue is that one is not named and the other one is, but you could have assigned a named function to a variable too.
However, there seems to be a difference on how they get assigned. The second one seems to get assigned to a variable that its named after, right at the start of the execution context. The first one has to wait for the explicit assignment within the execution context.
So you'd be testing for when they get assigned. You might think that's is possible to do so within the global object like:
//some protected vars that can't work without same-origin
var protected = ['caches', 'localStorage', 'sessionStorage', 'frameElement'];
var definedAtInit = [];
for(prop in window){
if(!isSandboxed(prop) && typeof window[prop] === 'function'){
definedAtInit.push(prop);
}
};
function isSandboxed(prop){
return protected.indexOf(prop) !== -1;
}
function isItDefinedAtInit(funcName){
return definedAtInit.indexOf(funcName) !== -1;
}
var func = function() {
console.log('test');
}
var results = { isItDefinedAtInit : isItDefinedAtInit('isItDefinedAtInit'),
func : isItDefinedAtInit('func')
};
document.getElementById('results').innerHTML = JSON.stringify(results, '/t');
<pre id="results"></pre>
However, you could still do something like:
var isItDefinedAtInit = function() { };
//After this, isItDefinedAtInit('isItDefinedAtInit') would be wrong.
And you still have the problems with other execution contexts, I don't think functions declared within a function execution context get attached to any object.
I think these kind of checks are a bad idea to be honest.
There is only way, we can determine function has defined with function declarations not as expression.
as Grundy mentioned name property of the respective function gives require information, if it has been defined with expression name property holds undefined value, else it holds function name.
Here is the code :
var isDefinedAsFunction = function(fn){
return fn.name !== undefined
}

Is it possible to change scope of function after new Function()?

So I have this code:
function fn(a){
var f=(new Function("return a"));
return f();
}
fn(7)//ReferenceError: a is not defined
Same problems with local variables:
function fn(){
var a=7;
var f=new Function("return a");
return f();
}
fn(7)//ReferenceError: a is not defined
I want it to return a but the new function cant see a,
it can only see global a
var a=1;
function fn(a){
var f=(new Function("return a"));
return f();
}
fn(7)//1
With the normal initialization the function can see the argument.
function fn(a){
var f=function(){return a};
return f();
}
fn(7)//7
I need to call the basic constructor in my project and can't use global variables.
I know that i could solve this by giving arguments to the newly created function and call it with that like this:
function fn(a){
var f=(new Function('a',"return a"));
return f(a);
}
fn(7)//7
And also could use some parsing function and some stupidly long method to make incoming arguments reachable like this:
function parsargs(funct){
//some parsing methodes giving back argument name list from funct.toString()
return "['a','b']";//like this
}
function fn(a,b){
var arrgstr,retfunc;
arrgstr="";
for(var i in arguments)
{
if(i<arguments.length-1)
arrgstr+=arguments[i]+",";
else
arrgstr+=arguments[i];
}
//return arrgstr;
retfunc="var f=new Function("+parsargs()+",'return b*a');return f("+arrgstr+")";
return (new Function(retfunc))();
}
fn(7,4)//28
But there must be an easier way which reaches local variables and functions as well...
Any suggestions?
PS:
i am trying to replace eval() in the project
Here is a simplified version of my original problem:
fiddle
The answer is NO...
Your exact question isn't clear but, supposing you can use arguments.callee (i.e. non strict mode) and that you want to be able to have any arguments name in fn, you "may" do this:
function fn(a,b){
var strargs = arguments.callee.toString().match(/\(([^\)]*)\)/)[1];
return (new Function(strargs.split(","),"return a+b")).apply(null,arguments);
}
console.log(fn(7, 3)) // 10
But I have a strong feeling this is a XY question and that we could have given a more useful answer knowing the real original problem to solve.
You could call your new Function with a context that references the local variables that you need:
function f(a) {
var b = 30;
return new Function("return this.a + this.b").call({ a: a, b: b })
}
f(10) // 40
The reason why this isn't working as easy as you would like is that JavaScript doesn't blindly put variables in a scope. Instead, it parses the function body (as good as it can) and determines which variables the code will need. It will then look up the variables in the outer scopes, create references for them and attach those references to the new function. That's what keeps variables from the outer scope accessible inside of the function when it eventually will be executed.
You want to build a function from a string body. JavaScript can't tell which variables the body will use and hence, it won't make those variables available.
To make it work for function arguments, use apply()::
function fn(a){
var argref = arguments; // create reference to the arguments of fn() and hence to a or any other arguments
var func = new Function("return arguments[0]");
var wrapper = function() {
return func.apply(null, argref);
};
return wrapper;
}
Note that you still can't reference arguments by name since the code never specifies what the names of the arguments of func are - JavaScript can't magically read you mind and do what you want. If you want to access the arguments by name, you need to tell the interpreter the names.
This question has some code how to determine the names from a function reference: How to get function parameter names/values dynamically from javascript
I don't see a way to make local variables available without passing them to fn() as arguments. Even if you used them inside of fn(), they would be out of scope when func() is eventually executed.
A simple solution would be to pass an object to fn():
function fn(conf) {
var func = new Function('conf', "return conf.a");
var wrapper = function(conf) {
return func.apply(null, conf);
};
return wrapper;
}
fn({a:7});
new Function doesn't create a closure context, but eval does.
Here's a low-tech way to build a function with an arbitrary evaluated body and access to the calling scope:
function f(a) {
return eval("(function() { return a })")()
}
f(10) // 10

Specifics of javascript cosntructors

Say I have a pure constructor function (containing nothing but this.Bar = bar)
1) When I call it from another function, can I pass the caller function's arguments directly when I call or must I do var myBar=new bar, myBar.Bar=thebar, where the bar is a caller argument?
2) Will the constructor still instantiate even if it doesn't get all the args?
3) How can I check if one of the args is unique, IE no other instance of the object has this value for the property in question? Specifically, I want to assign each object a unique index at creation. Maybe array?
Many thanks in advance
Say I have a pure constructor function (containing nothing but this.Bar = bar)
I'm going to assume you mean:
function MyConstructor(bar) {
this.Bar = bar;
}
(Note: The overwhelming convention in JavaScript is that property names start with a lower-case letter. So this.bar, not this.Bar. Initially-capped identifiers are usually reserved for constructor functions.)
1) When I call it from another function, can I pass the caller function's arguments directly when I call or must I do var myBar=new bar, myBar.Bar=thebar, where the bar is a caller argument?
You can pass them directly:
function foo(a, b, c) {
var obj = new MyConstructor(b);
}
2) Will the constructor still instantiate even if it doesn't get all the args?
The number of arguments passed is not checked by the JavaScript engine. Any formal arguments that you don't pass will have the value undefined when the function is called:
function MyConstructor(bar) {
console.log(bar);
}
var obj = new MyConstructor(); // logs "undefined"
3) How can I check if one of the args is unique, IE no other instance of the object has this value for the property in question? Specifically, I want to assign each object a unique index at creation. Maybe array?
In general, that's usually not in-scope for a constructor. But yes, you could use an array, or an object, to do that.
var knownBars = [];
function MyConstructor(bar) {
if (knownBars.indexOf(bar) !== -1) {
// This bar is known
}
else {
// Remember this bar
knownBars.push(bar);
}
}
Of course, indexOf may not be what you want for searching, so you may need to use some other method of Array.prototype or your own loop.
Another way would be to use an object; this assumes that bar is a string or something that can usefully be turned into a string:
var knownBars = {};
function MyConstructor(bar) {
if (knownBars.indexOf(bar) !== -1) {
// This bar is known
}
else {
// Remember this bar
knownBars[bar] = 1;
}
}
When I call it from another function, can I pass the caller function's arguments directly
There wouldn't be much point in having this.Bar = bar in the constructor function if you could not.
Will the constructor still instantiate even if it doesn't get all the args?
Assuming nothing inside it throws an exception if the argument is missing, yes. Arguments just get a value of undefined.
How can I check if one of the args is unique, IE no other instance of the object has this value for the property in question?
You'd need to have a shared store of them that you check against.
For example:
var Constructor = (function () {
var unique_check_store = {};
function RealConstructor(bar) {
if (typeof bar == 'undefined') {
throw "You must specify bar";
}
if (unique_check_store.hasOwnProperty(bar)) {
throw "You have already created one of these called '" + bar + "'";
}
this.Bar = bar;
unique_check_store[bar] = true;
}
return RealConstructor;
})();
var a, b, c;
try {
a = new Constructor();
} catch (e) {
alert(e);
}
try {
b = new Constructor("thing");
} catch (e) {
alert(e);
}
try {
c = new Constructor("thing");
} catch (e) {
alert(e);
}
alert(a);
alert(b);
alert(c);
​

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