What effect does the passing of primitive types into function.apply() or function.call() have on JavaScript's behavior?

Question

What effect does the passing of primitive types into function.apply() or function.call() have on JavaScript's behavior?

When a primitive type such as a string or number is used as the this subject in a function call (such as the first argument in either function.call() or function.apply()), the primitive type is converted to its object equivalent (for example, a string becomes a String).

For example:

var f = function(x) { return [typeof(this), typeof(x)]; }  
var obj = '123'  
f.call(obj, obj)  
>>> ["object", "string"]

This means that "this" becomes an object (specifically a String object) while the second argument passed to call remains a primitive string and serves as the first argument to the function "f".

Although both objects are "123", there are subtle differences (for instance, they are equal with "==", but not with "===").

I have observed this behavior in Chrome and Firefox, suggesting that there may be a specific reason for it. Despite my search efforts, I have yet to find any explanation. I would appreciate any clarification on why this conversion occurs, ideally accompanied by references to documentation explaining the relevant rules.

javascript

Answer 1

Answer №1

It appears that this behavior is correct.

According to Function.prototype.call, the called function receives ToObject(thisArg) as its this value.

ToObject "converts its argument to an Object type in the following way":

If it's a String - A new String object is created with [[value]] property set to the string's value.

Answer 2

It appears that this behavior is correct.

According to Function.prototype.call, the called function receives ToObject(thisArg) as its this value.

ToObject "converts its argument to an Object type in the following way":

If it's a String - A new String object is created with [[value]] property set to the string's value.

Answer 3

Answer №2

When calling a Javascript function using Call and Apply methods, the first parameter determines the context in which the function will run. It must always be an object.

Let's look at an example to better understand this concept:

function totest() 
{
    this.ff  = function(x) {
        this.test(x);
    };

    this.test = function(x) {
        alert(x);
    }

}

function totest1() 
{   
 this.test = function(x) {
    alert(x);
}

}

function fun() 
{
     var obj = new totest();
     var obj1 = new totest1();
     obj.ff('hi');  //Runs fine without any problem
     obj.ff.call(obj, 'sam') ;  //Runs fine without any problem
     obj.ff.call(this, 'sam');  //throws an error
     obj.ff.call(obj1, 'sam');  //will be executed in the context of totest1

}

Calling obj.ff.call(this, 'sam') results in an error. WHY?

This is because we are trying to execute the ff method within the context of fun() (or a window), not within the context of totest.

On the other hand, obj.ff.call(obj1, 'sam') specifies that the method should be executed in the context of totest1(), where it works since it has access to the test method.

Therefore, the first parameter in Call or Apply must be an object, while the remaining parameters define the arguments for the method being executed.

I hope this clarifies things for you!

Answer 4

When calling a Javascript function using Call and Apply methods, the first parameter determines the context in which the function will run. It must always be an object.

Let's look at an example to better understand this concept:

function totest() 
{
    this.ff  = function(x) {
        this.test(x);
    };

    this.test = function(x) {
        alert(x);
    }

}

function totest1() 
{   
 this.test = function(x) {
    alert(x);
}

}

function fun() 
{
     var obj = new totest();
     var obj1 = new totest1();
     obj.ff('hi');  //Runs fine without any problem
     obj.ff.call(obj, 'sam') ;  //Runs fine without any problem
     obj.ff.call(this, 'sam');  //throws an error
     obj.ff.call(obj1, 'sam');  //will be executed in the context of totest1

}

Calling obj.ff.call(this, 'sam') results in an error. WHY?

This is because we are trying to execute the ff method within the context of fun() (or a window), not within the context of totest.

On the other hand, obj.ff.call(obj1, 'sam') specifies that the method should be executed in the context of totest1(), where it works since it has access to the test method.

Therefore, the first parameter in Call or Apply must be an object, while the remaining parameters define the arguments for the method being executed.

I hope this clarifies things for you!

Answer 5

Answer №3

In a nutshell: The initial parameter is converted into an object if it is of a primitive type because, as you highlighted, it may be accessed by this within the invoked function. Since this must point to an object, the runtime environment ensures that there is indeed an object to reference. Consider how you would address this scenario, and you will likely reach the same conclusion. Great inquiry.

Side note: I commend Ramesh's detailed response, but felt compelled to provide a quick answer for those seeking immediate clarification.

Answer 6

In a nutshell: The initial parameter is converted into an object if it is of a primitive type because, as you highlighted, it may be accessed by this within the invoked function. Since this must point to an object, the runtime environment ensures that there is indeed an object to reference. Consider how you would address this scenario, and you will likely reach the same conclusion. Great inquiry.

Side note: I commend Ramesh's detailed response, but felt compelled to provide a quick answer for those seeking immediate clarification.

What effect does the passing of primitive types into function.apply() or function.call() have on JavaScript's behavior?

Answer №1

Answer №2

Answer №3

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