How to avoid object reference in javascript
One of the fundamental differences of objects versus primitives is that objects are stored and copied “by reference”, whereas primitive values: strings, numbers, booleans, etc – are always copied “as a whole value”. Show That’s easy to understand if we look a bit under the hood of what happens when we copy a value. Let’s start with a primitive, such as a string. Here we put a copy of
As a result we have two independent variables, each one storing the string Quite an obvious result, right? Objects are not like that. A variable assigned to an object stores not the object itself, but its “address in memory” – in other words “a reference” to it. Let’s look at an example of such a variable:
And here’s how it’s actually stored in memory: The object is stored somewhere in memory (at the right of the picture), while the We may think of an object variable, such as When we perform actions with the object, e.g. take a property Now here’s why it’s important. When an object variable is copied, the reference is copied, but the object itself is not duplicated. For instance:
Now we have two variables, each storing a reference to the same object: As you can see, there’s still one object, but now with two variables that reference it. We can use either variable to access the object and modify its contents:
It’s as if we had a cabinet with two keys and used one of them ( Comparison by referenceTwo objects are equal only if they are the same object. For instance, here
And here two independent objects are not equal, even though they look alike (both are empty):
For comparisons like Const objects can be modified An important side effect of storing objects as references is that an object declared as For instance:
It might seem that the line In other words, the That said, if we really need to make constant object properties, it’s also possible, but using totally different methods. We’ll mention that in the chapter Property flags and descriptors. Cloning and merging, Object.assignSo, copying an object variable creates one more reference to the same object. But what if we need to duplicate an object? We can create a new object and replicate the structure of the existing one, by iterating over its properties and copying them on the primitive level. Like this:
We can also use the method Object.assign. The syntax is:
For instance, we can use it to merge several objects into one:
If the copied property name already exists, it gets overwritten:
We also can use
It copies all properties of There are also other methods of cloning an object, e.g. using the spread syntax Nested cloningUntil now we assumed that all properties of Like this:
Now it’s not enough to copy
To fix that and make structuredCloneThe call Here’s how we can use it in our example:
The It also supports circular references, when an object property references the object itself (directly or via a chain or references). For instance:
As you can see, Although, there are cases when For instance, when an object has a function property:
Function properties aren’t supported. To handle such complex cases we may need to use a combination of cloning methods, write custom code or, to not reinvent the wheel, take an existing implementation, for instance _.cloneDeep(obj) from the JavaScript library lodash. SummaryObjects are assigned and copied by reference. In other words, a variable stores not the “object value”, but a “reference” (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object itself. All operations via copied references (like adding/removing properties) are performed on the same single object. To make a “real copy” (a clone) we can use How do you handle an object reference not set to an instance of an object?The best way to avoid the "NullReferenceException: Object reference not set to an instance of an object” error is to check the values of all variables while coding. You can also use a simple if-else statement to check for null values, such as if (numbers!= null) to avoid this exception.
Why are objects passed as reference JavaScript?Objects and Arrays
Objects in JavaScript are passed by reference. When more than one variable is set to store either an object , array or function , those variables will point to the same allocated space in the memory. Passed by reference.
Does JavaScript pass object by reference?JavaScript does not Pass By Reference. However, JavaScript does not make copies of objects when they are passed or assigned. As such, it is the same object with a different name - changes made to the object (from any name) affect said object. It depends on what you mean by "pass by reference".
What is object reference in JS?In JavaScript, objects are a reference type. Two distinct objects are never equal, even if they have the same properties. Only comparing the same object reference with itself yields true.
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