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What is the advantage of using call method instead of passing an object in the parameter

In the below code, which one is the right and how these two are different
Using call method
var obj = {
num: 10
};
var add = function(a) {
return this.num + a
}
console.log(add.call(obj,4))
Passing object in parameter
var obj = {
num: 10
};
var add = function(obj,a) {
return obj.num + a
}
console.log(add(obj,4))

Your second code block is just a regular function. The first one however is a bit more tricky. So the question is basically:
When to work with context in javascript?
In javascript, the term context basically means this. It is usually used when you call a method of an object, so that you can refer to the object. That's one of the core concepts of OOP, were we only define a function once inside the prototype, and every object of that class which inherits from it exposes this method, it won't work without context. So that's what this was invented for. However there are some cases, were context is useful without inheritance. E.g. Eventhandlers are usually contextless, as they are not part of any object:
window.addEventListener("load", function(evt){
const el = evt.target;
};
However as it is an Eventhandler of window, wouldn't it make sense that it is executed in the context of window? If you now say "YES", then you (will) probably love JS:
window.addEventListener("load", function(){
this.document.body.innerHTML = "Dynamic context can be cool!";
});
So in JS this is the way of refering to the object, the function refers to. Through Function.prototype.call we can make use of this everywhere. However that does not mean that we should use it everywhere. this should stay in the sense of context, as using it somewhere else will create confusion / uglify your code / make your code buggy.
var add = function(a) {
return this.num + a;
}
In your codesnippet i think its unclear what thisrefers to. So its rather a misuse of this. However it could get a meaning if you make it a method of obj, so its context becomes clear from the code:
const num = {
value:10,
add(a){ return this.value + a }
};
It gets even more beautiful if you use inheritance to make it reusable:
class CustomNumber {
constructor(n = 0){
this.value = n;
}
add(a){ return this.value + a; }
}
const num = new CustomNumber(10);

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();

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
}

Any way to get the currently executing function object in javascript?

Is there any way to refer to the function object that you're currently executing in? If it's not a method of any object or not called with .call() or .apply(), the this pointer is likely just window, not the function object.
I often use a design pattern like this for global variables that I want scoped to a particular function as this keeps them out of the top level namespace:
function generateRandom() {
if (!generateRandom.prevNums) {
generateRandom.prevNums = {}; // generateRandom.prevNums is a global variable
}
var random;
do {
random = Math.floor((Math.random() * (99999999 - 10000000 + 1)) + 10000000);
} while (generateRandom.prevNums[random])
generateRandom.prevNums[random] = true;
return(random.toString());
}
But, I'd rather not have to spell out the function name every time I want to use a variable scoped to that object. If the name of the function ever changes, there are then a lot of places to change the name.
Is there any way to get the currently executing function object?

Well, you could use arguments.callee()...
https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope/arguments/callee
From MDN:
Description
callee is a property of the arguments object. It can be used to refer
to the currently executing function inside the function body of that
function. This is for example useful when you don't know the name of
this function, which is for example the case with anonymous functions.
Note: You should avoid using arguments.callee() and just give every
function (expression) a name.
BUT...
What you really want are Javascript Prototypes.
function RandomSomethingGenerator()
{
this.prevNums = {};
}
RandomSomethingGenerator.prototype.generate = function() {
var random;
do {
random = Math.floor((Math.random() * (99999999 - 10000000 + 1)) + 10000000);
} while (this.prevNums[random])
this.prevNums[random] = true;
return(random.toString());
};
Why do I say this?
1.) You're dirtying the global space with all those functions.
2.) Even if you like Jani's suggestion, and you want a "static" function like you have now, my suggestion would be the same, but with a twist: Create your global function, and wrap an instance of an object (built from a prototype) inside the closure and make the call to it (so, basically, make yourself a singleton).
As in this (adapted from Jani's answer):
var randomSomething = (function() {
var randomSomethingGenerator = new RandomSomethingGenerator();
return function() {
randomSomethingGenerator.generate();
};
})();

I don't think there's any way to do exactly what you ask, but you could use a closure for your function-local static variables instead.
You can easily achieve this using an IIFE:
var generateRandom = (function() {
//any function's static variables go here
var prevNums = {};
return function() {
//function code goes here
var random;
do {
random = Math....
}
prevNums[random] = true;
return random.toString();
};
})();

You want arguments.callee. From MDN - callee:
callee is a property of the arguments object. It can be used to refer to the currently executing function inside the function body of that function. This is for example useful when you don't know the name of this function, which is for example the case with anonymous functions.
For example:
> foo = function() { console.log(arguments.callee); };
> bar = function() { foo() };
> bar();
function () { console.log(arguments.callee) }
However, I think this is being deprecated. The above link says, "The 5th edition of ECMAScript forbids use of arguments.callee() in strict mode."

Given a string describing a Javascript function, convert it to a Javascript function

Say I've got a Javascript string like the following
var fnStr = "function(){blah1;blah2;blah3; }" ;
(This may be from an expression the user has typed in, duly sanitized, or it may be the result of some symbolic computation. It really doesn't matter).
I want to define fn as if the following line was in my code:
var fn = function(){blah1;blah2;blah3; } ;
How do I do that?
The best I've come up with is the following:
var fn = eval("var f = function(){ return "+fnStr+";}; f() ;") ;
This seems to do the trick, even though it uses the dreaded eval(), and uses a slightly convoluted argument. Can I do better? I.e. either not use eval(), or supply it with a simpler argument?

There's also the Function object.
var adder = new Function("a", "b", "return a + b");

You can do this:
//in your case: eval("var fn = " + fnStr);
eval("var fn = function(){ blah1;blah2;blah3; }");
fn();
Not sure how to get it much simpler, sometimes there's no (better) way around eval(). Here's a quick example of this in action.

Use parentheses.
var fn = eval("(function() {...})");
This technique is also good for transmitting JSON values.
By the way, it's often better to build functions by composing them directly from other functions. If you are using strings, you have to worry about things like unexpected variable capture.

Here's what I use for simple cases:
// an example function
function plus(...args) {
return args.reduce( (s,v) => s+v, 0 );
}
// function to string
let str = plus.toString();
// string to function
let copy = new Function('return ' + str)();
// tests
console.assert(plus.name == 'plus');
console.assert(copy.name == 'plus');
console.assert(plus.constructor == Function);
console.assert(copy.constructor == Function);
console.assert(plus(1,2,3,4) === copy(1,2,3,4));
console.assert(plus.toString() === copy.toString());

You can also insert the string into a script element and then insert the script element into the page.
script_ele = window.document.createElement("script");
script_ele.innerHTML = 'function my_function(){alert("hi!");}';
window.document.body.appendChild(script_ele);
my_function();

One way:
var a = 'function f(){ alert(111); } function d(){ alert(222);}';
eval(a);
d();
A second more secure way to convert string to a function:
// function name and parameters to pass
var fnstring = "runMe";
var fnparams = ["aaa", "bbbb", "ccc"];
// find object
var fn = window[fnstring];
// is object a function?
if (typeof fn === "function") fn.apply(null, fnparams);
function runMe(a,b){
alert(b);
}
Look at the working code: http://plnkr.co/edit/OiQAVd9DMV2PfK0NG9vk

The Function constructor creates a new Function object. In JavaScript every function is actually a Function object.
// Create a function that takes two arguments and returns the sum of those arguments
var fun = new Function("a", "b", "return a + b");
// Call the function
fun(2, 6);
Output: 8

You can call Parse your string as javascript fuction
function getDate(){alert('done')} // suppose this is your defined function
to call above function getDate() is this in string format like 'getDate()'
var callFunc=new Function('getDate()') //Parse and register your function
callFunc() // Call the function