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how to access the value in a function which is defined inside a key of an object in javascript

i have passed the value 'hp' to the key function get() and can access it in the next line when it log it to console and then i proceed to pass the same variable to another function set() which is defined inside of the key function get() but the value is undefined when i try to access it in log statement of the inner function.
note- If i dont pass the variable company inside set() i can access it.
I have just started learning JS so any links would be appreciated.
var laptop = {
company: "none",
get: function(company) {
console.log("outer", company);
var set = function(company) {
console.log("inner", company);
};
set();
}
};
laptop.get("hp");

I think what you're looking for is something like the following:
var laptop = {
company: "none",
get: function(company) {
// use `this` to access the object's properties
console.log("outer", this.company);
var set = function(arg) {
console.log("inner", arg);
// alternatively, just use `company`
// because of `closures`, which give you
// access to an outer function's scrope
// from an inner function
console.log("inner #2", company);
};
set(company);
}
};
laptop.get("hp");
However, I'd strongly advise against using the same name for your variables/arguments/parameters inside the same scope, the way you have it in your example, to avoid confusion.
Learn more about closures in JavaScript:
JavaScript.info - Variable scope

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

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

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;
};
}
};

closure not working

var Dog = function() {
var _instance = 'hello world';
return function() {
console.log(this._instance);
}
} (); //note that it is self invoking function
var l = new Dog(); //#> undefined
In the above case I was expecting an output of:
'hello world'
Why is this._instance not accessing the the variable which should be accessible by virtue of closure? I tested this in FF and am getting undefined.

You don't assign _instance to the object, it's just a closure variable, and should be accessed without using this:
var Dog = function() {
var _instance = 'hello world';
return function() {
console.log(_instance);
}
} (); //note that it is self invoking function
var l = new Dog();
I'd probably write it like so instead:
var Dog = (function() {
var defaults = {
name: 'Rags'
};
var Dog = function (options) {
// Take options as a constructor argument, this
// allows you to merge it with defaults to override
// options on specific instances
this.setOptions(options);
};
Dog.prototype = {
// Common methods for Dogs here
setOptions: function (options) {
// Declare all variables in the beginning of the method because
// JavaScript hoists variable declarations
var key = null;
// First assign all the defaults to this object
for ( key in defaults) {
this[key] = defaults[key];
}
// Now override with the values in options:
if (options && options.hasOwnProperty) {
for ( key in options ) {
this[key] = options[key];
}
}
}
};
return Dog; // Return the constructor method
} ()); // wrap the self-invoked function in paranthesis to visualize that
// it creates a closure
var buster = new Dog({name: 'Buster'}),
unnamed = new Dog();
alert(buster.name); // Alerts 'Buster'
alert(unnamed.name); // Alerts 'Rags'
Note that I have not tried to compile the above code, so it might contain a few mistakes. Nothing JsLint can't handle though!
You might want to consider adding filtering to the setOptions method so that it doesn't assign properties you don't want, or filter out methods etc declared in the options-parameter.
Additionally, if you use JQuery, or similar library, there are (often) utility functions for merging objects, making it trivial to write the setOptions-method:
function setOptions (options) {
// I assume JQuery here
// true as the first argument gives us a recursive merge
var mergedOptions = $.extend(true, defaults, options);
for (var key in mergedOptions ) {
if(this.checkAllowedProperty(key, typeof(mergedOptions[key])) {
this[key] = mergedOptions[key];
}
}
}
/**
* This method checks if propertyName is an allowed property on this object.
* If dataType is supplied it also checks if propertyName is allowed for
* dataType
* #return true if propertyName, with type dataType, is allowed on this object,
* else false
*/
function checkAllowedProperty (propertyName, dataType);

Your problem is this.

Change this._instance to _instance. You may also want to wrap your self-invoking function in parentheses like (function() { ... })(); for maximum browser compatibility.

As the others have said, you need to remove "this." from your function.
The reason for the problem is down to the binding of the "this" keyword in the two function contexts. Inside the closure, "this" refers to the function that is being returned, and not to the outer function. You could resolve this by doing the following:
var Dog = function() {
var _instance = 'hello world';
var that = this; //Assign "this" to "that"
return function() {
console.log(that._instance); //Use reference to "that"
}
} ();
var l = new Dog();
You could also probably do something closer with the function.apply() method, but I'll leave that to you.
I hope that helps.

Perhaps you are satisfied by removing "this.", but you may be interested to learn that "this" doesn't refer to what you wanted it to anyway. What it refers to really depends on how the function is called. It does not necessarily refer to an instance of an object constructed by the function you returned, or its container function, or to any other object. By default, if you merely call the function as a normal function, "this" will refer to the global window context.
What you must do to have "this" be bound to any specific object is to call the function as a method of that object, or of its prototype. e.g. foo.myMethod(). Another way is that you can use the apply or call method, passing in the object you want it to apply to. e.g. anyFunction.apply(foo).