Unusual quirks in javascript variables when used with arrays

Question

Unusual quirks in javascript variables when used with arrays

Unsure if this question has been asked previously. Nevertheless, I couldn't find it anywhere. I've observed a peculiar behavior that seems to occur only with arrays.

Here is the typical behavior I anticipate from variables:

var k = 10,
    m = k;
alert(m); //10
k+=1;
alert(m); //10

Now, observe how they behave when dealing with arrays.

var arr = [],
    arr2 = arr;
alert(arr2); // empty
arr.push(1);
alert(arr2); // 1

It appears that with arrays, variables are merely references to the same array, while with numbers, they signify two distinct numbers that happen to have the same value.

I apologize if this seems like a beginner's question, but I recently made this observation. Does this apply to all complex types? Also, what is the rationale behind this behavior? What purpose does the language serve by implementing it in this manner?

javascript arrays variables

Answer 1

Answer №1

The initial state of the first code block shows:

m = k;

This line copies the value from variable k to variable m:

k+=1 is executed, it only changes the value in variable

k</code, leaving variable <code>m

unaffected:

arr and arr2 are references to arrays located elsewhere in memory.

Hence, pushing a value results in the following transformation:

+------+
| arr  |------+
+------+      |   +-----------+
              +-->| (array )  |
+------+      |   +-----------+
| arr2 |------+   | length: 0 |
+------+          | 0: 1      |
                  +-----------+

Does this apply to all complex types?

Indeed, this behavior applies to all objects (including standard arrays which are objects in JavaScript) as well as new typed arrays.

The key concept to remember is that variables store values. When a variable points to an object or array, the stored value is a reference to the actual object/array, not a duplicate of it.

Understanding that object references are treated as values, similar to numbers, greatly aids in comprehending JavaScript code (as well as code in other languages that operate in a similar manner, such as Java).

Answer 2

The initial state of the first code block shows:

m = k;

This line copies the value from variable k to variable m:

k+=1 is executed, it only changes the value in variable

k</code, leaving variable <code>m

unaffected:

arr and arr2 are references to arrays located elsewhere in memory.

Hence, pushing a value results in the following transformation:

+------+
| arr  |------+
+------+      |   +-----------+
              +-->| (array )  |
+------+      |   +-----------+
| arr2 |------+   | length: 0 |
+------+          | 0: 1      |
                  +-----------+

Does this apply to all complex types?

Indeed, this behavior applies to all objects (including standard arrays which are objects in JavaScript) as well as new typed arrays.

The key concept to remember is that variables store values. When a variable points to an object or array, the stored value is a reference to the actual object/array, not a duplicate of it.

Understanding that object references are treated as values, similar to numbers, greatly aids in comprehending JavaScript code (as well as code in other languages that operate in a similar manner, such as Java).

Answer 3

Answer №2

Let's delve into why this phenomenon occurs:

Javascript categorizes values into two main types:

Complex types
Primitive types

An object is considered a complex type in Javascript, and examples include:

[]
{}
new String()
new Date()

In contrast, primitive types consist of:

'a string'
23
true

The reason for the difference lies in how referencing works for complex versus primitive types.

var aString = 'myString';
var refString = aString;

In the above scenario, "aString" is simply copied to refString, creating two independent values. However, when it comes to complex types:

var anArray = [];
var refArray = anArray;

var anObject = {};
var refObject = anObject;

Both variables reference the same array or object instance, rather than separate copies.

This concept also applies when checking for equality:

'my string' === 'my string' // true
23 === 23 // true
true === true // true

For complex types, Javascript doesn't compare memory references but rather value similarities:

{} === {} // false
[] === [] // false

var myObject = {};
var myObjectReferenced = myObject;
myObject === myObjectReferenced // true

Understanding this distinction is crucial in Javascript to avoid inadvertently altering shared objects thinking they are unique.

I hope this explanation clarifies things for you.

Answer 4

Let's delve into why this phenomenon occurs:

Javascript categorizes values into two main types:

Complex types
Primitive types

An object is considered a complex type in Javascript, and examples include:

[]
{}
new String()
new Date()

In contrast, primitive types consist of:

'a string'
23
true

The reason for the difference lies in how referencing works for complex versus primitive types.

var aString = 'myString';
var refString = aString;

In the above scenario, "aString" is simply copied to refString, creating two independent values. However, when it comes to complex types:

var anArray = [];
var refArray = anArray;

var anObject = {};
var refObject = anObject;

Both variables reference the same array or object instance, rather than separate copies.

This concept also applies when checking for equality:

'my string' === 'my string' // true
23 === 23 // true
true === true // true

For complex types, Javascript doesn't compare memory references but rather value similarities:

{} === {} // false
[] === [] // false

var myObject = {};
var myObjectReferenced = myObject;
myObject === myObjectReferenced // true

Understanding this distinction is crucial in Javascript to avoid inadvertently altering shared objects thinking they are unique.

I hope this explanation clarifies things for you.

Answer 5

Answer №3

When it comes to array objects, they are interconnected through their pointer variables. Therefore, any changes made in one object will directly impact the other object as well.

Answer 6

When it comes to array objects, they are interconnected through their pointer variables. Therefore, any changes made in one object will directly impact the other object as well.

Answer 7

Answer №4

In JavaScript, numbers are considered to be primitive data types, which means that the actual data is stored directly in the variables themselves.

var x = 5,
    y = x;

When assigning a value, such as in y = x, a copy of the value is created for each variable. Any changes made to one variable will not affect the other because they are independent copies.

On the contrary, arrays are treated as reference types in JavaScript.

var arr = [],
    arrCopy = arr;

When an array is assigned to another variable, like arrCopy = arr, it actually copies a reference to the original array. This means that any modifications made to the array through one variable will be reflected in the other variable as well.

Answer 8

In JavaScript, numbers are considered to be primitive data types, which means that the actual data is stored directly in the variables themselves.

var x = 5,
    y = x;

When assigning a value, such as in y = x, a copy of the value is created for each variable. Any changes made to one variable will not affect the other because they are independent copies.

On the contrary, arrays are treated as reference types in JavaScript.

var arr = [],
    arrCopy = arr;

When an array is assigned to another variable, like arrCopy = arr, it actually copies a reference to the original array. This means that any modifications made to the array through one variable will be reflected in the other variable as well.

Answer 9

Answer №5

In JavaScript (as well as in other programming languages), there exist two types of variables. The first type is known as "primitive" types, which include numbers, strings, and booleans. On the other hand, the second type is referred to as "reference" types, encompassing all other object types.

A helpful analogy is to envision primitive types as individual drawers, each containing a specific value. For instance, one drawer may hold the number 1, while another stores the string "John". When accessing a primitive type, you directly retrieve the value from its designated drawer.

Contrastingly, reference types do not actually store the value within the drawer itself. Instead, they hold instructions on how to locate the value in memory, which remains hidden. If you duplicate the contents of a "reference drawer" into another variable, you are duplicating the directions to access the value rather than the value itself. Consequently, both variables would point to the same underlying value.

The rationale behind this distinction involves efficient memory management within your program, optimizing resources effectively.

Answer 10

In JavaScript (as well as in other programming languages), there exist two types of variables. The first type is known as "primitive" types, which include numbers, strings, and booleans. On the other hand, the second type is referred to as "reference" types, encompassing all other object types.

A helpful analogy is to envision primitive types as individual drawers, each containing a specific value. For instance, one drawer may hold the number 1, while another stores the string "John". When accessing a primitive type, you directly retrieve the value from its designated drawer.

Contrastingly, reference types do not actually store the value within the drawer itself. Instead, they hold instructions on how to locate the value in memory, which remains hidden. If you duplicate the contents of a "reference drawer" into another variable, you are duplicating the directions to access the value rather than the value itself. Consequently, both variables would point to the same underlying value.

The rationale behind this distinction involves efficient memory management within your program, optimizing resources effectively.

Unusual quirks in javascript variables when used with arrays

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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