The elimination function in JavaScript

Question

The elimination function in JavaScript

I've been browsing the Mozilla Developers website, focusing on the concept of the delete operator. In the final section about "Deleting array elements," there are two similar scripts presented, with the only difference being how they modify the array.

Looking at the first script, I find it puzzling why the "if" statement does not execute. As far as I understand, the delete operator is supposed to "remove the element of the array." So, if I were to type trees[3] in the console, I would expect to see undefined.

var trees = ["redwood","bay","cedar","oak","maple"];
delete trees[3];
if (3 in trees) {
    // this does not get executed
}

Now, in the second script, it appears to be trying to mimic the delete operation, but not exactly. Here, undefined is assigned to trees[3]. What baffles me is how the "if" block runs in this script while the first example does not. Can someone shed some light on this JavaScript behavior for me?

var trees = ["redwood","bay","cedar","oak","maple"];
trees[3] = undefined;
if (3 in trees) {
    // this gets executed
}

javascript arrays

Answer 1

Answer №1

When looking at the two approaches you are experimenting with, there is a significant contrast between them:

Approach 1:

In this method, you are eliminating, erasing, and completely getting rid of the element 3 in your array named tree. This means that no trace of 3 in tree remains, and consequently, the if statement returns false.

Approach 2:

Contrary to the first method, in this approach, you are assigning a new value, specifically undefined, to the element with the key 3 in the array. Despite this change, there still exists 3 in tree, leading to the if check resulting in true.

Answer 2

When looking at the two approaches you are experimenting with, there is a significant contrast between them:

Approach 1:

In this method, you are eliminating, erasing, and completely getting rid of the element 3 in your array named tree. This means that no trace of 3 in tree remains, and consequently, the if statement returns false.

Approach 2:

Contrary to the first method, in this approach, you are assigning a new value, specifically undefined, to the element with the key 3 in the array. Despite this change, there still exists 3 in tree, leading to the if check resulting in true.

Answer 3

Answer №2

When looking at your second example, it's important to note that the key 3 is still present, but its value is undefined. This aspect of JavaScript can be confusing, but it's just a characteristic of the language. The in operator specifically checks for the existence of a key, not whether the value is defined.

If you were to output the entire arrays after each "deletion," the first example would show:

["redwood", "bay", "cedar", 4: "maple"]

On the other hand, the second example would display something like this:

["redwood", "bay", "cedar", undefined, "maple"]

As demonstrated, in the first example, the key is entirely absent, and the iteration moves on to the next key, 4. In contrast, in the second example, the key still exists, but its value is set to undefined.

Answer 4

When looking at your second example, it's important to note that the key 3 is still present, but its value is undefined. This aspect of JavaScript can be confusing, but it's just a characteristic of the language. The in operator specifically checks for the existence of a key, not whether the value is defined.

If you were to output the entire arrays after each "deletion," the first example would show:

["redwood", "bay", "cedar", 4: "maple"]

On the other hand, the second example would display something like this:

["redwood", "bay", "cedar", undefined, "maple"]

As demonstrated, in the first example, the key is entirely absent, and the iteration moves on to the next key, 4. In contrast, in the second example, the key still exists, but its value is set to undefined.

Answer 5

Answer №3

It is important to understand the distinction between user-defined undefined values and those returned by the javascript engine to signify non-existence.

For example, consider the scenario where you have an array of trees:

var trees = ["redwood", "bay", "cedar", "oak", "maple"];
trees[3] = undefined;
if (3 in trees) {
    console.log("hi");
}

In this case, javascript recognizes that the element at index 3 exists, but the user has specifically set it to undefined.

Now, let's look at a different scenario:

if (5 in trees) {
    console.log("hi");
}

Here, the property at index 5 was never created in the array. Javascript understands this lack of creation as a true non-existence, and therefore does not display "hi" as the property does not exist.

Answer 6

It is important to understand the distinction between user-defined undefined values and those returned by the javascript engine to signify non-existence.

For example, consider the scenario where you have an array of trees:

var trees = ["redwood", "bay", "cedar", "oak", "maple"];
trees[3] = undefined;
if (3 in trees) {
    console.log("hi");
}

In this case, javascript recognizes that the element at index 3 exists, but the user has specifically set it to undefined.

Now, let's look at a different scenario:

if (5 in trees) {
    console.log("hi");
}

Here, the property at index 5 was never created in the array. Javascript understands this lack of creation as a true non-existence, and therefore does not display "hi" as the property does not exist.

Answer 7

Answer №4


    if(3 in tree) {
        //Some code that will not be executed
    }

is indeed accurate. The way the in operator works in JavaScript is different from Python. It simply checks if an object has a property. For example, an array in JavaScript has a property 0, similar to how a string has a property 2 with the value someString[2]. The distinction between delete object[prop]; and object[prop] = undefined; can be observed by using object.hasOwnProperty(prop); or by iterating through the values or properties of the object.

Answer 8


    if(3 in tree) {
        //Some code that will not be executed
    }

is indeed accurate. The way the in operator works in JavaScript is different from Python. It simply checks if an object has a property. For example, an array in JavaScript has a property 0, similar to how a string has a property 2 with the value someString[2]. The distinction between delete object[prop]; and object[prop] = undefined; can be observed by using object.hasOwnProperty(prop); or by iterating through the values or properties of the object.

The elimination function in JavaScript

Answer №1

Approach 1:

Approach 2:

Answer №2

Answer №3

Answer №4

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