Running the JavaScript call() function and implementing the Prototype - Slice Function

Question

Running the JavaScript call() function and implementing the Prototype - Slice Function

Currently, I am diving into the MDN Article regarding the slice method in JavaScript. With everything making sense except for the 2nd example found under the section labeled Array-Like Objects.

In that particular example, it mentions simplifying the initial case by creating our own version of slice as shown below:

var unboundSlice = Array.prototype.slice;
var slice = Function.prototype.call.bind(unboundSlice);

function list() {
  return slice(arguments);
}

var list1 = list(1, 2, 3); // [1, 2, 3]

The point of confusion arises from the fact that call appears directly after prototype on the second line.

Usually, we see it structured more like

Array.prototype.slice.call(arguments)

or something along those lines.

The intricacies of how the first two lines interact to ultimately create a functioning slice function elude me at the moment.

javascript call slice apply

Answer 1

Answer №1

In summary:

var slice = Function.prototype.call.bind(unboundSlice);

can be condensed as follows:

var slice = function(value, start, end) {
  return unboundSlice.call(value, start, end);
};

Let's take a moment to analyze this line:

Array.prototype.slice.call(arguments)

The .slice method is used to extract part of an array and works on the this value. The .call method allows us to specify the this value for a function execution. By using the above line, we can execute slice on arguments without modifying arguments directly. Alternatively, we could have written

arguments.slice = Array.prototype.slice;
arguments.slice();

but this approach is less elegant.

Now let's examine

Function.prototype.call.bind(unboundSlice);

As mentioned, .call is a universal method for all functions that sets the this value. It operates on this, which should be a function. The call method effectively does the following:

function call(thisValue, arg1, arg2, ...) {
   return this.apply(thisValue, [arg1, arg2, ...]);
}

Take note of how it calls this like a function.

.bind is another common method for functions. It creates a new function with a fixed this value based on the first argument provided.

What would the resulting function from call.bind(unboundSlice) look like?

function boundCall(thisValue, arg1, arg2, ...) {
   return unboundSlice.apply(thisValue, [arg1, arg2, ...]);
}

We merely swapped out this with unboundSlice. Now, boundCall will always invoke unboundSlice.

Answer 2

In summary:

var slice = Function.prototype.call.bind(unboundSlice);

can be condensed as follows:

var slice = function(value, start, end) {
  return unboundSlice.call(value, start, end);
};

Let's take a moment to analyze this line:

Array.prototype.slice.call(arguments)

The .slice method is used to extract part of an array and works on the this value. The .call method allows us to specify the this value for a function execution. By using the above line, we can execute slice on arguments without modifying arguments directly. Alternatively, we could have written

arguments.slice = Array.prototype.slice;
arguments.slice();

but this approach is less elegant.

Now let's examine

Function.prototype.call.bind(unboundSlice);

As mentioned, .call is a universal method for all functions that sets the this value. It operates on this, which should be a function. The call method effectively does the following:

function call(thisValue, arg1, arg2, ...) {
   return this.apply(thisValue, [arg1, arg2, ...]);
}

Take note of how it calls this like a function.

.bind is another common method for functions. It creates a new function with a fixed this value based on the first argument provided.

What would the resulting function from call.bind(unboundSlice) look like?

function boundCall(thisValue, arg1, arg2, ...) {
   return unboundSlice.apply(thisValue, [arg1, arg2, ...]);
}

We merely swapped out this with unboundSlice. Now, boundCall will always invoke unboundSlice.

Answer 3

Answer №2

After reading the helpful MDN article on Function.prototype.call(), I finally grasped the concept.

To put it simply:

In JavaScript, every Function has a method called call. Since functions are objects, they inherit properties and methods from their prototype.

For instance, when you use

Array.prototype.slice.call(arguments)

, you're essentially calling the call method on the slice function.

The line

var slice = Function.prototype.call.bind(unboundSlice);

demonstrates the call method associated with the Function prototype.

If you're still unsure about prototypes in JavaScript, check out JavaScript Prototypes.

1 Functions act as objects.

2 "Every JavaScript object has a prototype."

3 "The prototype itself is an object."

4 "All properties and methods of JavaScript objects are inherited from their prototype."

So, to sum it up: In JavaScript, a Function comes with a method known as call.

And for a better understanding of what bind does, the comparison between that = this and .bind discussed in this article might clarify things.

If all else fails, make sure you grasp the concepts of context and scope.

Answer 4

After reading the helpful MDN article on Function.prototype.call(), I finally grasped the concept.

To put it simply:

In JavaScript, every Function has a method called call. Since functions are objects, they inherit properties and methods from their prototype.

For instance, when you use

Array.prototype.slice.call(arguments)

, you're essentially calling the call method on the slice function.

The line

var slice = Function.prototype.call.bind(unboundSlice);

demonstrates the call method associated with the Function prototype.

If you're still unsure about prototypes in JavaScript, check out JavaScript Prototypes.

1 Functions act as objects.

2 "Every JavaScript object has a prototype."

3 "The prototype itself is an object."

4 "All properties and methods of JavaScript objects are inherited from their prototype."

So, to sum it up: In JavaScript, a Function comes with a method known as call.

And for a better understanding of what bind does, the comparison between that = this and .bind discussed in this article might clarify things.

If all else fails, make sure you grasp the concepts of context and scope.

Answer 5

Answer №3

In JavaScript, the slice property belongs to Array.prototype. It requires its this object to be Array-like. When dealing with Array-like objects that lack their own slice function, you can utilize it by:

Array.prototype.slice.call(arraylikething);

To simplify this process and avoid excessive typing, a function can be created to achieve the same outcome:

var slice = function(arraylikething){
    return Array.prototype.slice.call(arraylikething);
};

For ease of use, JavaScript offers Function.prototype.bind which allows functions to be bound to a specific this object. This enables us to attain the same result more conveniently:

var slice = Function.prototype.call.bind(Array.prototype.slice);

The bind function generates a new function that executes call with Array.prototype.slice set as its this object. This is equivalent to manually setting it as done above, and is identical to your original code.

Answer 6

In JavaScript, the slice property belongs to Array.prototype. It requires its this object to be Array-like. When dealing with Array-like objects that lack their own slice function, you can utilize it by:

Array.prototype.slice.call(arraylikething);

To simplify this process and avoid excessive typing, a function can be created to achieve the same outcome:

var slice = function(arraylikething){
    return Array.prototype.slice.call(arraylikething);
};

For ease of use, JavaScript offers Function.prototype.bind which allows functions to be bound to a specific this object. This enables us to attain the same result more conveniently:

var slice = Function.prototype.call.bind(Array.prototype.slice);

The bind function generates a new function that executes call with Array.prototype.slice set as its this object. This is equivalent to manually setting it as done above, and is identical to your original code.

Answer 7

Answer №4

Chris Dillinger's response was not only accurate but also insightful. However, there is another perspective to consider when approaching this problem. Essentially, you are tasked with defining

Function.prototype.call.bind(Array.prototype.slice)

This can be viewed in the following manner:

fn.bind(context) 
    ==>  function(...args) {return context.fn(...args);} 
            // 1. Explanation of `bind` (simplified for understanding)

fn.bind(unboundSlice) 
    ==>  function(...args) {return unboundSlice.fn(...args);}  
            // 2. Replacing `context` with `unboundSlice`

Function.prototype.call.bind(unboundSlice)
    ==>  function(...args) {return unboundSlice[Function.prototype.call](...args);} 
            // 3. Swapping `fn` with `Function.prototype.call`

Function.prototype.call.bind(unboundSlice)
    ==>  function(...args) {return unboundSlice[.call(...args);} 
            // 4. Tracing the prototype chain

Function.prototype.call.bind(Array.prototype.slice)
    ==>  function(...args) {return Array.prototype.slice.call(...args);}
            // 5. Replacing `unboundSlice` with `Array.prototype.slice`

The most complex step here is step 4, where one must understand that all functions inherit the call method from their prototype chain. Therefore, invoking call on them is essentially another way of calling the functions themselves.

Answer 8

Chris Dillinger's response was not only accurate but also insightful. However, there is another perspective to consider when approaching this problem. Essentially, you are tasked with defining

Function.prototype.call.bind(Array.prototype.slice)

This can be viewed in the following manner:

fn.bind(context) 
    ==>  function(...args) {return context.fn(...args);} 
            // 1. Explanation of `bind` (simplified for understanding)

fn.bind(unboundSlice) 
    ==>  function(...args) {return unboundSlice.fn(...args);}  
            // 2. Replacing `context` with `unboundSlice`

Function.prototype.call.bind(unboundSlice)
    ==>  function(...args) {return unboundSlice[Function.prototype.call](...args);} 
            // 3. Swapping `fn` with `Function.prototype.call`

Function.prototype.call.bind(unboundSlice)
    ==>  function(...args) {return unboundSlice[.call(...args);} 
            // 4. Tracing the prototype chain

Function.prototype.call.bind(Array.prototype.slice)
    ==>  function(...args) {return Array.prototype.slice.call(...args);}
            // 5. Replacing `unboundSlice` with `Array.prototype.slice`

The most complex step here is step 4, where one must understand that all functions inherit the call method from their prototype chain. Therefore, invoking call on them is essentially another way of calling the functions themselves.

Answer 9

Answer №5

Initially, in the first sentence, the method Array.prototype.slice is simply referenced as unboundSlice. This action involves 'extracting' the slice method from Array.prototype.

In the following line, a similar process occurs for the method Function.prototype.call, which is applicable to ALL functions because it is defined within Function.prototype and inherited by all functions.

By using .bind(unboundSlice), the Call function's this value becomes bound to the reference of Array.prototype.slice. This essentially results in a similar outcome to Array.prototype.slice.call(), where call also has its this set to slice due to it being a method of slice and called in that manner.

Ultimately, the bound call method is then referred to as var slice;

The main concept here is the ability to utilize an array method's functionality (such as slice) in a different context, specifically the global scope.

Instead of directly calling call when it is already a method of slice, you are binding slice to serve as the this value of call to achieve the same result.

Answer 10

Initially, in the first sentence, the method Array.prototype.slice is simply referenced as unboundSlice. This action involves 'extracting' the slice method from Array.prototype.

In the following line, a similar process occurs for the method Function.prototype.call, which is applicable to ALL functions because it is defined within Function.prototype and inherited by all functions.

By using .bind(unboundSlice), the Call function's this value becomes bound to the reference of Array.prototype.slice. This essentially results in a similar outcome to Array.prototype.slice.call(), where call also has its this set to slice due to it being a method of slice and called in that manner.

Ultimately, the bound call method is then referred to as var slice;

The main concept here is the ability to utilize an array method's functionality (such as slice) in a different context, specifically the global scope.

Instead of directly calling call when it is already a method of slice, you are binding slice to serve as the this value of call to achieve the same result.

Running the JavaScript call() function and implementing the Prototype - Slice Function

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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