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The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name , and has members for performing common array operations .
Description
In JavaScript, arrays aren't primitives but are instead Array objects with the following core characteristics:
- JavaScript arrays are resizable and can contain a mix of different data types . (When those characteristics are undesirable, use typed arrays instead.)
- JavaScript arrays are not associative arrays and so, array elements cannot be accessed using arbitrary strings as indexes, but must be accessed using nonnegative integers (or their respective string form) as indexes.
- JavaScript arrays are zero-indexed : the first element of an array is at index 0 , the second is at index 1 , and so on — and the last element is at the value of the array's length property minus 1 .
- JavaScript array-copy operations create shallow copies . (All standard built-in copy operations with any JavaScript objects create shallow copies, rather than deep copies ).
Array indices
Array objects cannot use arbitrary strings as element indexes (as in an associative array ) but must use nonnegative integers (or their respective string form). Setting or accessing via non-integers will not set or retrieve an element from the array list itself, but will set or access a variable associated with that array's object property collection . The array's object properties and list of array elements are separate, and the array's traversal and mutation operations cannot be applied to these named properties.
Array elements are object properties in the same way that toString is a property (to be specific, however, toString() is a method). Nevertheless, trying to access an element of an array as follows throws a syntax error because the property name is not valid:
JavaScript syntax requires properties beginning with a digit to be accessed using bracket notation instead of dot notation . It's also possible to quote the array indices (e.g., years['2'] instead of years[2] ), although usually not necessary.
The 2 in years[2] is coerced into a string by the JavaScript engine through an implicit toString conversion. As a result, '2' and '02' would refer to two different slots on the years object, and the following example could be true :
Only years['2'] is an actual array index. years['02'] is an arbitrary string property that will not be visited in array iteration.
Relationship between length and numerical properties
A JavaScript array's length property and numerical properties are connected.
Several of the built-in array methods (e.g., join() , slice() , indexOf() , etc.) take into account the value of an array's length property when they're called.
Other methods (e.g., push() , splice() , etc.) also result in updates to an array's length property.
When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array's length property accordingly:
Increasing the length .
Decreasing the length property does, however, delete elements.
This is explained further on the Array/length page.
Array methods and empty slots
Empty slots in sparse arrays behave inconsistently between array methods. Generally, the older methods will skip empty slots, while newer ones treat them as undefined .
Among methods that iterate through multiple elements, the following do an in check before accessing the index and do not conflate empty slots with undefined :
- copyWithin()
- lastIndexOf()
- reduceRight()
For exactly how they treat empty slots, see the page for each method.
These methods treat empty slots as if they are undefined :
- findIndex()
- findLastIndex()
- group() Experimental
- groupToMap() Experimental
- toLocaleString()
Copying methods and mutating methods
Some methods do not mutate the existing array that the method was called on, but instead return a new array. They do so by first accessing this.constructor[Symbol.species] to determine the constructor to use for the new array. The newly constructed array is then populated with elements. The copy always happens shallowly — the method never copies anything beyond the initially created array. Elements of the original array(s) are copied into the new array as follows:
- Objects: the object reference is copied into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays.
- Primitive types such as strings, numbers and booleans (not String , Number , and Boolean objects): their values are copied into the new array.
Other methods mutate the array that the method was called on, in which case their return value differs depending on the method: sometimes a reference to the same array, sometimes the length of the new array.
The following methods create new arrays with @@species :
- splice() (to construct the array of removed elements that's returned)
Note that group() and groupToMap() do not use @@species to create new arrays for each group entry, but always use the plain Array constructor. Conceptually, they are not copying methods either.
The following methods mutate the original array:
Iterative methods
Many array methods take a callback function as an argument. The callback function is called sequentially and at most once for each element in the array, and the return value of the callback function is used to determine the return value of the method. They all share the same signature:
Where callbackFn takes three arguments:
The current element being processed in the array.
The index of the current element being processed in the array.
The array that the method was called upon.
What callbackFn is expected to return depends on the array method that was called.
The thisArg argument (defaults to undefined ) will be used as the this value when calling callbackFn . The this value ultimately observable by callbackFn is determined according to the usual rules : if callbackFn is non-strict , primitive this values are wrapped into objects, and undefined / null is substituted with globalThis . The thisArg argument is irrelevant for any callbackFn defined with an arrow function , as arrow functions don't have their own this binding.
All iterative methods are copying and generic , although they behave differently with empty slots .
The following methods are iterative:
- groupToMap()
In particular, every() , find() , findIndex() , findLast() , findLastIndex() , and some() do not always invoke callbackFn on every element — they stop iteration as soon as the return value is determined.
There are two other methods that take a callback function and run it at most once for each element in the array, but they have slightly different signatures from typical iterative methods (for example, they don't accept thisArg ):
The sort() method also takes a callback function, but it is not an iterative method. It mutates the array in-place, doesn't accept thisArg , and may invoke the callback multiple times on an index.
Generic array methods
Array methods are always generic — they don't access any internal data of the array object. They only access the array elements through the length property and the indexed elements. This means that they can be called on array-like objects as well.
Normalization of the length property
The length property is converted to an integer and then clamped to the range between 0 and 2 53 - 1. NaN becomes 0 , so even when length is not present or is undefined , it behaves as if it has value 0 .
Some array methods set the length property of the array object. They always set the value after normalization, so length always ends as an integer.
Array-like objects
The term array-like object refers to any object that doesn't throw during the length conversion process described above. In practice, such object is expected to actually have a length property and to have indexed elements in the range 0 to length - 1 . (If it doesn't have all indices, it will be functionally equivalent to a sparse array .)
Many DOM objects are array-like — for example, NodeList and HTMLCollection . The arguments object is also array-like. You can call array methods on them even if they don't have these methods themselves.
Constructor
Creates a new Array object.
Static properties
Returns the Array constructor.
Static methods
Creates a new Array instance from an array-like object or iterable object.
Returns true if the argument is an array, or false otherwise.
Creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.
Instance properties
These properties are defined on Array.prototype and shared by all Array instances.
The constructor function that created the instance object. For Array instances, the initial value is the Array constructor.
Contains property names that were not included in the ECMAScript standard prior to the ES2015 version and that are ignored for with statement-binding purposes.
These properties are own properties of each Array instance.
Reflects the number of elements in an array.
Instance methods
Returns the array item at the given index. Accepts negative integers, which count back from the last item.
Returns a new array that is the calling array joined with other array(s) and/or value(s).
Copies a sequence of array elements within an array.
Returns a new array iterator object that contains the key/value pairs for each index in an array.
Returns true if every element in the calling array satisfies the testing function.
Fills all the elements of an array from a start index to an end index with a static value.
Returns a new array containing all elements of the calling array for which the provided filtering function returns true .
Returns the value of the first element in the array that satisfies the provided testing function, or undefined if no appropriate element is found.
Returns the index of the first element in the array that satisfies the provided testing function, or -1 if no appropriate element was found.
Returns the value of the last element in the array that satisfies the provided testing function, or undefined if no appropriate element is found.
Returns the index of the last element in the array that satisfies the provided testing function, or -1 if no appropriate element was found.
Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.
Returns a new array formed by applying a given callback function to each element of the calling array, and then flattening the result by one level.
Calls a function for each element in the calling array.
Groups the elements of an array into an object according to the strings returned by a test function.
Groups the elements of an array into a Map according to values returned by a test function.
Determines whether the calling array contains a value, returning true or false as appropriate.
Returns the first (least) index at which a given element can be found in the calling array.
Joins all elements of an array into a string.
Returns a new array iterator that contains the keys for each index in the calling array.
Returns the last (greatest) index at which a given element can be found in the calling array, or -1 if none is found.
Returns a new array containing the results of invoking a function on every element in the calling array.
Removes the last element from an array and returns that element.
Adds one or more elements to the end of an array, and returns the new length of the array.
Executes a user-supplied "reducer" callback function on each element of the array (from left to right), to reduce it to a single value.
Executes a user-supplied "reducer" callback function on each element of the array (from right to left), to reduce it to a single value.
Reverses the order of the elements of an array in place . (First becomes the last, last becomes first.)
Removes the first element from an array and returns that element.
Extracts a section of the calling array and returns a new array.
Returns true if at least one element in the calling array satisfies the provided testing function.
Sorts the elements of an array in place and returns the array.
Adds and/or removes elements from an array.
Returns a localized string representing the calling array and its elements. Overrides the Object.prototype.toLocaleString() method.
Returns a string representing the calling array and its elements. Overrides the Object.prototype.toString() method.
Adds one or more elements to the front of an array, and returns the new length of the array.
Returns a new array iterator object that contains the values for each index in the array.
An alias for the values() method by default.
This section provides some examples of common array operations in JavaScript.
Note: If you're not yet familiar with array basics, consider first reading JavaScript First Steps: Arrays , which explains what arrays are , and includes other examples of common array operations.
Create an array
This example shows three ways to create new array: first using array literal notation , then using the Array() constructor, and finally using String.prototype.split() to build the array from a string.
Create a string from an array
This example uses the join() method to create a string from the fruits array.
Access an array item by its index
This example shows how to access items in the fruits array by specifying the index number of their position in the array.
Find the index of an item in an array
This example uses the indexOf() method to find the position (index) of the string "Banana" in the fruits array.
Check if an array contains a certain item
This example shows two ways to check if the fruits array contains "Banana" and "Cherry" : first with the includes() method, and then with the indexOf() method to test for an index value that's not -1 .
Append an item to an array
This example uses the push() method to append a new string to the fruits array.
Remove the last item from an array
This example uses the pop() method to remove the last item from the fruits array.
Note: pop() can only be used to remove the last item from an array. To remove multiple items from the end of an array, see the next example.
Remove multiple items from the end of an array
This example uses the splice() method to remove the last 3 items from the fruits array.
Truncate an array down to just its first N items
This example uses the splice() method to truncate the fruits array down to just its first 2 items.
Remove the first item from an array
This example uses the shift() method to remove the first item from the fruits array.
Note: shift() can only be used to remove the first item from an array. To remove multiple items from the beginning of an array, see the next example.
Remove multiple items from the beginning of an array
This example uses the splice() method to remove the first 3 items from the fruits array.
Add a new first item to an array
This example uses the unshift() method to add, at index 0 , a new item to the fruits array — making it the new first item in the array.
Remove a single item by index
This example uses the splice() method to remove the string "Banana" from the fruits array — by specifying the index position of "Banana" .
Remove multiple items by index
This example uses the splice() method to remove the strings "Banana" and "Strawberry" from the fruits array — by specifying the index position of "Banana" , along with a count of the number of total items to remove.
Replace multiple items in an array
This example uses the splice() method to replace the last 2 items in the fruits array with new items.
Iterate over an array
This example uses a for...of loop to iterate over the fruits array, logging each item to the console.
But for...of is just one of many ways to iterate over any array; for more ways, see Loops and iteration , and see the documentation for the every() , filter() , flatMap() , map() , reduce() , and reduceRight() methods — and see the next example, which uses the forEach() method.
Call a function on each element in an array
This example uses the forEach() method to call a function on each element in the fruits array; the function causes each item to be logged to the console, along with the item's index number.
Merge multiple arrays together
This example uses the concat() method to merge the fruits array with a moreFruits array, to produce a new combinedFruits array. Notice that fruits and moreFruits remain unchanged.
Copy an array
This example shows three ways to create a new array from the existing fruits array: first by using spread syntax , then by using the from() method, and then by using the slice() method.
All built-in array-copy operations ( spread syntax , Array.from() , Array.prototype.slice() , and Array.prototype.concat() ) create shallow copies . If you instead want a deep copy of an array, you can use JSON.stringify() to convert the array to a JSON string, and then JSON.parse() to convert the string back into a new array that's completely independent from the original array.
You can also create deep copies using the structuredClone() method, which has the advantage of allowing transferable objects in the source to be transferred to the new copy, rather than just cloned.
Finally, it's important to understand that assigning an existing array to a new variable doesn't create a copy of either the array or its elements. Instead the new variable is just a reference, or alias, to the original array; that is, the original array's name and the new variable name are just two names for the exact same object (and so will always evaluate as strictly equivalent ). Therefore, if you make any changes at all either to the value of the original array or to the value of the new variable, the other will change, too:
Grouping the elements of an array
The Array.prototype.group() methods can be used to group the elements of an array, using a test function that returns a string indicating the group of the current element.
Here we have a simple inventory array that contains "food" objects that have a name and a type .
To use group() , you supply a callback function that is called with the current element, and optionally the current index and array, and returns a string indicating the group of the element.
The code below uses an arrow function to return the type of each array element (this uses object destructuring syntax for function arguments to unpack the type element from the passed object). The result is an object that has properties named after the unique strings returned by the callback. Each property is assigned an array containing the elements in the group.
Note that the returned object references the same elements as the original array (not deep copies ). Changing the internal structure of these elements will be reflected in both the original array and the returned object.
If you can't use a string as the key, for example, if the information to group is associated with an object that might change, then you can instead use Array.prototype.groupToMap() . This is very similar to group except that it groups the elements of the array into a Map that can use an arbitrary value ( object or primitive ) as a key.
Creating a two-dimensional array
The following creates a chessboard as a two-dimensional array of strings. The first move is made by copying the 'p' in board[6][4] to board[4][4] . The old position at [6][4] is made blank.
Here is the output:
Using an array to tabulate a set of values
Creating an array using the result of a match.
The result of a match between a RegExp and a string can create a JavaScript array that has properties and elements which provide information about the match. Such an array is returned by RegExp.prototype.exec() and String.prototype.match() .
For example:
For more information about the result of a match, see the RegExp.prototype.exec() and String.prototype.match() pages.
Specifications
Browser compatibility.
BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.
- Accessing properties
- Indexed collections
- Typed Arrays
- RangeError: invalid array length
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assign object in array javascript
i'm trying to assign object in array. for example:
so, i want the output gonna be like: cat1 cat2 cat3 etc
- 1 What is the problem here? – Evan Knowles May 16, 2014 at 5:36
- its error if write the code like that – Leo ibel May 16, 2014 at 5:37
- 1 People will generally help you faster if you post the error. – Evan Knowles May 16, 2014 at 5:39
4 Answers 4
The problem is in this block:
You're assigning values to indices of the array that don't exist yet.
This appends the "cat" variable to the end of the array.
- Not .name isn't defined (it wouldn't matter, we assign to it), but catties[i] . You don't need to use push btw, assigning to catties[i] would be fine as well. – Bergi May 16, 2014 at 6:00
- You're correct; been switching between Java and JS too much today. Fixing. – Caleb Brinkman May 16, 2014 at 6:02
If you want to push objects inside array, you need to construct an object first and then you can push it.
As per your code, you have only pushed the value.
So use this :
Here is a Fiddle that does what you need. "names" object is not defined:
http://jsfiddle.net/DianaNassar/94tVw/
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JS Tutorial
Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript arrays.
An array is a special variable, which can hold more than one value:
Why Use Arrays?
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can access the values by referring to an index number.
Creating an Array
Using an array literal is the easiest way to create a JavaScript Array.
It is a common practice to declare arrays with the const keyword.
Learn more about const with arrays in the chapter: JS Array Const .
Spaces and line breaks are not important. A declaration can span multiple lines:
You can also create an array, and then provide the elements:
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
The two examples above do exactly the same.
There is no need to use new Array() .
For simplicity, readability and execution speed, use the array literal method.
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Accessing Array Elements
You access an array element by referring to the index number :
Note: Array indexes start with 0.
[0] is the first element. [1] is the second element.
Changing an Array Element
This statement changes the value of the first element in cars :
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Arrays are Objects
Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for arrays.
But, JavaScript arrays are best described as arrays.
Arrays use numbers to access its "elements". In this example, person[0] returns John:
Objects use names to access its "members". In this example, person.firstName returns John:
Array Elements Can Be Objects
JavaScript variables can be objects. Arrays are special kinds of objects.
Because of this, you can have variables of different types in the same Array.
You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and methods:
Array methods are covered in the next chapters.
The length Property
The length property of an array returns the length of an array (the number of array elements).
The length property is always one more than the highest array index.
Accessing the First Array Element
Accessing the last array element, looping array elements.
One way to loop through an array, is using a for loop:
You can also use the Array.forEach() function:
Adding Array Elements
The easiest way to add a new element to an array is using the push() method:
New element can also be added to an array using the length property:
Adding elements with high indexes can create undefined "holes" in an array:
Associative Arrays
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes .
WARNING !! If you use named indexes, JavaScript will redefine the array to an object.
After that, some array methods and properties will produce incorrect results .
Example:
The difference between arrays and objects.
In JavaScript, arrays use numbered indexes .
In JavaScript, objects use named indexes .
Arrays are a special kind of objects, with numbered indexes.
When to Use Arrays. When to use Objects.
- JavaScript does not support associative arrays.
- You should use objects when you want the element names to be strings (text) .
- You should use arrays when you want the element names to be numbers .
JavaScript new Array()
JavaScript has a built-in array constructor new Array() .
But you can safely use [] instead.
These two different statements both create a new empty array named points:
These two different statements both create a new array containing 6 numbers:
The new keyword can produce some unexpected results:
A Common Error
is not the same as:
How to Recognize an Array
A common question is: How do I know if a variable is an array?
The problem is that the JavaScript operator typeof returns " object ":
The typeof operator returns object because a JavaScript array is an object.
Solution 1:
To solve this problem ECMAScript 5 (JavaScript 2009) defined a new method Array.isArray() :
Solution 2:
The instanceof operator returns true if an object is created by a given constructor:
Complete Array Reference
For a complete Array reference, go to our:
Complete JavaScript Array Reference .
The reference contains descriptions and examples of all Array properties and methods.
Test Yourself With Exercises
Get the value " Volvo " from the cars array.
Start the Exercise
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JavaScript Array of Objects Tutorial – How to Create, Update, and Loop Through Objects Using JS Array Methods
On average I work with JSON data 18 times a week. And I still need to google for specific ways to manipulate them almost every time. What if there was an ultimate guide that could always give you the answer?
In this article, I'll show you the basics of working with object arrays in JavaScript.
If you ever worked with a JSON structure, you've worked with JavaScript objects. Quite literally. JSON stands for JavaScript Object Notation.
Creating an object is as simple as this:
This object represents a car. There can be many types and colors of cars, each object then represents a specific car.
Now, most of the time you get data like this from an external service. But sometimes you need to create objects and their arrays manually. Like I did when I was creating this e-shop:
Considering each category list item looks like this in HTML:
I didn't want to have this code repeated 12 times, which would make it unmaintainable.
Creating an array of objects
But let's get back to cars. Let's take a look at this set of cars:
We can represent it as an array this way:
Arrays of objects don't stay the same all the time. We almost always need to manipulate them. So let's take a look at how we can add objects to an already existing array.
Add a new object at the start - Array.unshift
To add an object at the first position, use Array.unshift .
Add a new object at the end - Array.push
To add an object at the last position, use Array.push .
Add a new object in the middle - Array.splice
To add an object in the middle, use Array.splice . This function is very handy as it can also remove items. Watch out for its parameters:
So if we want to add the red Volkswagen Cabrio on the fifth position, we'd use:
Looping through an array of objects
Let me ask you a question here: Why do you want to loop through an array of objects? The reason I'm asking is that the looping is almost never the primary cause of what we want to achieve.
JavaScript provides many functions that can solve your problem without actually implementing the logic in a general cycle. Let's take a look.
Find an object in an array by its values - Array.find
Let's say we want to find a car that is red. We can use the function Array.find .
This function returns the first matching element:
It's also possible to search for multiple values:
let car = cars.find(car => car.color === "red" && car.type === "cabrio");
In that case we'll get the last car in the list.
Get multiple items from an array that match a condition - Array.filter
The Array.find function returns only one object. If we want to get all red cars, we need to use Array.filter .
Transform objects of an array - Array.map
This is something we need very often. Transform an array of objects into an array of different objects. That's a job for Array.map . Let's say we want to classify our cars into three groups based on their size.
It's also possible to create a new object if we need more values:
Add a property to every object of an array - Array.forEach
But what if we want the car size too? In that case we can enhance the object for a new property size . This is a good use-case for the Array.forEach function.
Sort an array by a property - Array.sort
When we're done with transforming the objects, we usually need to sort them one way or another.
Typically, the sorting is based on a value of a property every object has. We can use the Array.sort function, but we need to provide a function that defines the sorting mechanism.
Let's say we want to sort the cars based on their capacity in descending order.
The Array.sort compares two objects and puts the first object in the second place if the result of the sorting function is positive. So you can look at the sorting function as if it was a question: Should the first object be placed in second place?
Make sure to always add the case for zero when the compared value of both objects is the same to avoid unnecessary swaps.
Checking if objects in array fulfill a condition - Array.every, Array.includes
Array.every and Array.some come handy when we just need to check each object for a specific condition.
Do we have a red cabrio in the list of cars? Are all cars capable of transporting at least 4 people? Or more web-centric: Is there a specific product in the shopping cart?
You may remember the function Array.includes which is similar to Array.some , but works only for primitive types.
In this article, we went through the basic functions that help you create, manipulate, transform, and loop through arrays of objects. They should cover most cases you will stumble upon.
If you have a use-case that requires more advanced functionality, take a look at this detailed guide to arrays or visit the W3 schools reference .
Or get in touch with me and I will prepare another article :-)
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Understanding Object.assign() Method in JavaScript
The Object.assign() method invokes the getters on the source objects and setters on the target object. It assigns properties only, not copying or defining new properties.
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Here is how the syntax of Object.assign() looks like:
Cloning an object
The following example demonstrates how you can use Object.assign() to clone an object:
Remember that Object.assign() only creates a shallow clone of the object and not a deep clone.
Merging objects
If the source objects have same properties , they are overwritten by the properties of the objects later in the parameters order:
Converting an array to an object
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The code idea have is something like this const combined = names.map ( (map)=> Object.assign ( {}, country, /*something to extract name from nested array names*/), {country} ) Any suggestions how i could spread the object in the nested array to form the desired output? If the code could be improved in other ways, please let me know as well
The Object.assign () method only copies enumerable and own properties from a source object to a target object. 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.
The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name, and has members for performing common array operations. Description In JavaScript, arrays aren't primitives but are instead Array objects with the following core characteristics:
You're assigning values to indices of the array that don't exist yet. for (var i=0; i<5; i++) { var cat = { name: "cat" + i }; catties.push (cat); } or for (var i=0; i<5; i++) { var cat = { name: "cat" + i }; catties [i] = cat; } This appends the "cat" variable to the end of the array. Share Improve this answer Follow
Using an array literal is the easiest way to create a JavaScript Array. Syntax: const array_name = [ item1, item2, ... ]; It is a common practice to declare arrays with the const keyword. Learn more about const with arrays in the chapter: JS Array Const. Example const cars = ["Saab", "Volvo", "BMW"]; Try it Yourself »
To add an object at the first position, use Array.unshift. let car = { "color": "red", "type": "cabrio", "registration": new Date ('2016-05-02'), "capacity": 2 } cars.unshift (car); Add a new object at the end - Array.push To add an object at the last position, use Array.push.
There are four ways to convert an array to an object in JavaScript: Using the Object.assign () method to convert an array to an object ,e.g. const obj = Object.assign ( {}, arr). Using the spread syntax ( ...) to convert an array to an object, e.g. const obj = {...arr}.
The Object.assign () method invokes the getters on the source objects and setters on the target object. It assigns properties only, not copying or defining new properties. The properties in the target object are overwritten by the properties in source objects if they have the same key.