Which internal function is triggered in JavaScript when I retrieve the value of an array element by its index?

Question

Which internal function is triggered in JavaScript when I retrieve the value of an array element by its index?

Check out this fascinating wtfjs code snippet:

var a = [,];
alert(a.indexOf(a[0]));

This example highlights the difference between uninitialized and undefined values:

The array a contains only one uninitialized element.

Accessing a[0] returns undefined.

Since a does not explicitly include the undefined value, a.indexOf(a[0]) === -1 evaluates to true.

But have you ever wondered why a[0] returns undefined? What internal mechanism is at play here?

P.S. Remember that undefined is a primitive type in JavaScript. The term uninitialized refers to a value without any specified JavaScript type, which, technically speaking, does not exist as a specific primitive type in JavaScript.

javascript

Answer 1

Answer №1

According to the ES5 specification, array initializations have a specific rule:

Elided array elements do not have a defined value.

It's important to note that these elements are considered as not defined, rather than simply being assigned the value of undefined. Even though they may appear empty, elided elements still impact the length of the array:

...the absence of an element in the array affects its length and shifts the index of subsequent elements.

When using the indexOf method on an array, one particular step is crucial:

Let kPresent represent the outcome of the internal [[HasProperty]] method with O as its argument and the array index k converted to a string.

Here, k refers to a number corresponding to an array index, while O represents the array itself. Because elided elements were never declared, the array lacks a property for the associated index.

Answer 2

According to the ES5 specification, array initializations have a specific rule:

Elided array elements do not have a defined value.

It's important to note that these elements are considered as not defined, rather than simply being assigned the value of undefined. Even though they may appear empty, elided elements still impact the length of the array:

...the absence of an element in the array affects its length and shifts the index of subsequent elements.

When using the indexOf method on an array, one particular step is crucial:

Let kPresent represent the outcome of the internal [[HasProperty]] method with O as its argument and the array index k converted to a string.

Here, k refers to a number corresponding to an array index, while O represents the array itself. Because elided elements were never declared, the array lacks a property for the associated index.

Answer 3

Answer №2

The .indexOf() function only looks at elements in an array that have been set explicitly. In this scenario, even though the array's length is 1 (or 2 in certain browsers), no elements have been explicitly set, resulting in an effective length of zero.

Another way to understand this concept:

var a = [,];
console.log(a.length); // 1 (in Firefox)
console.log('0' in a); // false

This reveals that despite the array having a length of 1, there is no element with an index of 0. Therefore, any attempt to access the value of a[0] will return undefined.

Now, let's experiment further:

a[0] = undefined;
console.log(a.length); // still 1
console.log('0' in a); // true !!

Once a property has been referenced on the left side of an assignment, it becomes "real," even if its value is now explicitly set as undefined.

For more information on the internal methods at play, you can refer to the Reference Specification type, specifically how its "Get" operation functions.

Answer 4

The .indexOf() function only looks at elements in an array that have been set explicitly. In this scenario, even though the array's length is 1 (or 2 in certain browsers), no elements have been explicitly set, resulting in an effective length of zero.

Another way to understand this concept:

var a = [,];
console.log(a.length); // 1 (in Firefox)
console.log('0' in a); // false

This reveals that despite the array having a length of 1, there is no element with an index of 0. Therefore, any attempt to access the value of a[0] will return undefined.

Now, let's experiment further:

a[0] = undefined;
console.log(a.length); // still 1
console.log('0' in a); // true !!

Once a property has been referenced on the left side of an assignment, it becomes "real," even if its value is now explicitly set as undefined.

For more information on the internal methods at play, you can refer to the Reference Specification type, specifically how its "Get" operation functions.

Answer 5

Answer №3

It's a bit of a puzzle. Take, for example, the array a = [,,1]. Upon inspection, you'll find that only index 2 contains a value, which is 1. Indexes 0 and 1 are completely devoid of any value; they have been implicitly assigned as undefined. In simpler terms, they are NOT DEFINED. When searching for a value of undefined in an array, the result will always be -1. However, if you explicitly set them to null instead, you'll see a different outcome.

To tackle your question directly, the reason why a.indexOf(a[0]) returns -1 is because a[0] is undefined.

Answer 6

It's a bit of a puzzle. Take, for example, the array a = [,,1]. Upon inspection, you'll find that only index 2 contains a value, which is 1. Indexes 0 and 1 are completely devoid of any value; they have been implicitly assigned as undefined. In simpler terms, they are NOT DEFINED. When searching for a value of undefined in an array, the result will always be -1. However, if you explicitly set them to null instead, you'll see a different outcome.

To tackle your question directly, the reason why a.indexOf(a[0]) returns -1 is because a[0] is undefined.

Answer 7

Answer №4

In an array, any elements that have not been assigned a value (including null) are considered undefined.

For example, if you create an array like this:

var x = [,null,];

The elements x[0] and x[2] will be undefined. Accessing any index in the array x will return undefined, except for x[1], which is null. The length property of an array is one more than the highest non-undefined index. For instance:

var y = [];
y[10] = null;
y.length
> 11

However, only y[10] will not return undefined.

To learn more about this topic, check out: http://msdn.microsoft.com/en-us/library/d8ez24f2(v=vs.94).aspx

If you're referring to uninitialized values, in JavaScript, they are represented by undefined.

Answer 8

In an array, any elements that have not been assigned a value (including null) are considered undefined.

For example, if you create an array like this:

var x = [,null,];

The elements x[0] and x[2] will be undefined. Accessing any index in the array x will return undefined, except for x[1], which is null. The length property of an array is one more than the highest non-undefined index. For instance:

var y = [];
y[10] = null;
y.length
> 11

However, only y[10] will not return undefined.

To learn more about this topic, check out: http://msdn.microsoft.com/en-us/library/d8ez24f2(v=vs.94).aspx

If you're referring to uninitialized values, in JavaScript, they are represented by undefined.

Which internal function is triggered in JavaScript when I retrieve the value of an array element by its index?

Answer №1

Answer №2

Answer №3

Answer №4

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Which internal function is triggered in JavaScript when I retrieve the value of an array element by its index?

Answer №1

Answer №2

Answer №3

Answer №4

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