Take a look at this snippet of JavaScript code:
var x = [1, 2, 3],
y = x;
y[1] = 3;
x; // x === [1, 3, 3] Why does this happen?
Why is the value of "x" changing when I update "y[1]"? I tried it in both Firefox and Chrome and got the same result. This doesn't happen with a simple number. Is this the expected behavior?
var foo = 1,
bar = foo;
bar = 3;
foo; // foo === 1 Great!
The array remains the same (as it is an object, it holds the same reference), in order to make changes to them individually, you must create a duplicate by using .slice() (which generates a new array with the elements copied at the first level), as shown below:
let x = [4, 5, 6],
y = x.slice();
y[1] = 5;
x; // x === [4, 5, 6]
Due to the fact that "a" and "b" are pointing to the identical array, there is no duplicate array created. When the value of "a" is assigned to "b", it actually assigns the reference to the array, rather than a copy of the array.
With numbers, it's different as you're working with primitive types. There is no method available to update the value on a Number instance.
A similar scenario can be observed with Date instances:
var d1 = new Date(), d2 = d1;
d1.setMonth(11); d1.setDate(25);
alert(d2.toString()); // alerts the date for Christmas day
Aside from the responses already provided, another way to duplicate an array is by utilizing the slice function:
let newArr = oldArr.slice(0)
In JavaScript, all objects are passed by reference, so if you want to create a copy of an object, you need to duplicate the entire object rather than just assigning it.
Creating a copy of an array is straightforward. You can achieve this by using the following code:
var a = [1, 2, 3];
var b = a.slice(0);
The slice(0) method in this code snippet will return a copy of the array starting from index 0 to the end of the array.
For more information, you can visit this link.
Understanding the distinction between Value types and Reference types is crucial in programming.
Value types are stored directly on the computer's "Stack," meaning you have the actual value itself, not just a pointer or reference to it.
On the other hand, Reference types do not contain the actual value of the object or variable but instead refer to where the value can be found.
Imagine two Reference Type variables pointing to the same value - any changes made to one will also affect the other since they both point to the same location.
However, when you copy the value from one variable to another, you create separate copies that are independent of each other, so changes to one will not impact the other.
Arrays serve as pointers to the specific memory location where they are held.
When you see the code x = y, it's a unique statement in that it instructs the variable x to reference the object y.
Most other operations involve changing the object that x is pointing to.
b = a;
b[1] = 3;
So, the first line here makes b point to the same array as a.
Then, the second line changes the array that both b and a are referencing.
When assigning an array, a reference to the existing array is used instead of creating a new one. However, numbers and boolean values are duplicated when assigned to a different variable.
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