The concat method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array. The source for this interactive example is stored in a GitHub repository.
A new Array instance. The concat method creates a new array consisting of the elements in the object on which it is called, followed in order by, for each argument, the elements of that argument if the argument is an array or the argument itself if the argument is not an array. It does not recurse into nested array arguments.
The concat method does not alter this or any of the arrays provided as arguments but instead returns a shallow copy that contains copies of the same elements combined from the original arrays.
Elements of the original arrays are copied into the new array as follows:. Furthermore, any operation on the new array except operations on elements which are object references will have no effect on the original arrays, and vice versa. Get the latest and greatest from MDN delivered straight to your inbox. Sign in to enjoy the benefits of an MDN account. The compatibility table in this page is generated from structured data.
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WebView Android Full support 1. Chrome Android Full support February 13, 9 min read When working with functional programming a good rule of thumb is to always create new objects instead of changing old ones. How exactly can we be sure that the changes we make to an object do not affect the code elsewhere? Removing the unwanted references altogether seems like a good idea. There are many ways to do this and each of them yields a slightly different result.
We are going to take a look at the most popular ones: shallow copydeep copymerging and assigning. Also, on top of listing the pros and cons of every approach, we are going to compare these variations in terms of their performance.
I am also going to provide links to the production-ready equivalents to use in an actual, real-life application. If you wish to see the entire code of a given solution just click on a title. The link will redirect you to the Github repository.
To shallow copy, an object means to simply create a new object with the exact same set of properties. We call the copy shallow because the properties in the target object can still hold references to those in the source object. As we can see, the first version with the spread operator is faster. This is likely due to the spread operator having been optimized for this use specifically. Click here to run the tests yourself. Shallow copying should be used whenever we want to lose a reference to the source object but hardly care about references to any nested properties, e.
When we make a deep copy we create a completely new object which holds no references to the original. Our first implementation works recursively. We write a deep function, which checks the type of the argument sent to it and either calls an appropriate function for the argument being an array or an object or simply returns the value of the argument if it is neither an array nor an object. The deepObject function takes all of the keys of an object and iterates over them, recursively calling the deep function for each value.
In the example, we have a user, he has registered the addresses. One address is the main one, which we can imagine to be the home address, and a second alternative address which we can consider the address of a relative.
Just by looking at it, I know I'm creating a new array and I'm not manipulating the existing array. Whereas if I look at Version Bit appears like I'm adding the trucks array to the cars array, and it doesn't seem obvious to me that the cars array isn't being changed. But, maybe that's just me. I'd be curious to know what you think? I prefer using spreadbecause I find it more concise and easier to write. BUT, there are still benefits of using concat.
Spread is fantastic when you know beforehand that you're dealing with arrays. But what happens when the source is something else, like a string. And you want to add that string to the array. Let's walk through an example. So it doesn't achieve the result we want. I know some of you are like, duh! I'll just write some conditional to make sure what I'm passing is an array and execute accordingly. Sure that'd work too. So here's the quick rule.
If you know you're dealing with arrays, use spread. Some of you are asking, hey, can't I also use push to merge an array.
And yes, you sure can! But when you use pushit manipulates or changes the existing array. It does NOT create a new array. So depending on what you're trying to do. Make sure you keep that in mind.
Also, when you're trying to push an array to another array. You will need to spread it, otherwise, you will end up getting a nested array.
It returns the target object. The source for this interactive example is stored in a GitHub repository.Zt 0350 clone
Properties in the target object are overwritten by properties in the sources if they have the same key. Later sources' properties overwrite earlier ones. It uses [[Get]] on the source and [[Set]] on the target, so it will invoke getters and setters. Therefore it assigns properties, versus copying or defining new properties.
This may make it unsuitable for merging new properties into a prototype if the merge sources contain getters. For copying property definitions including their enumerability into prototypes, use Object. Both String and Symbol properties are copied. In case of an error, for example if a property is non-writable, a TypeError is raised, and the target object is changed if any properties are added before the error is raised. For deep cloning, we need to use alternatives, because Object. The properties are overwritten by other objects that have the same properties later in the parameters order.
The destructured version becomes:. For an object inside another object, like:. Alright, so to access property values two levels deep, first wrap the original property inside the top level object in this case props in curly braces, then, inside that object, wrap the inner object, matchin another set of curly braces. For a slightly more complex object, like:.
The new destructured version becomes:. Once more, stepping through each tier: props gets wrapped in the first set of curly braces, matchthe object contained within props is wrapped in the second set of curly braces, and finally, the third object params with the value I want to access is wrapped in the third set of curly braces.
Each object contains some key values and arrays. You can notice in the following example we have created an array and then inside an array, we defined another array:.
So if we want to access the car type of Vitz which exists in index zero in our array car. Here is the demonstration below:. Here we first target our array name which is the car and then target index zero because our Vitz car type exists at index zero and then we target index 1.
So if we want to access the color of the civic car then we first target car array and then pass that car 1 index because civic exist in the second object and then we target civic array to access the color of the civic car.
We have around eight objects in total.Robustscaler
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