javascript syntax arrays terminals variables. 3 JavaScript Array Secrets You Might Not Know About
At learning JavaScript objects, we all come across phrases like “ Arrays are simple objects in Javascript". Today I want to take a closer look at this statement:
View example
Looking at the example above, it becomes obvious that an array is an object type. But what does this mean?
If you're not familiar with the typeof operator, you can learn more about it here.
Inheritance
To understand the difference between JavaScript's work with objects and arrays, let's look at the principle of inheritance.
Each object contains a reference to the parent (prototype) object. When you call a method, JavaScript will start looking for it in the object you are working with. If the method is not found, then the search for the prototype will begin. The search is carried out along the entire prototype chain until a method is found or the root object is reached.
View example
The example above creates a person object with own parameter name. When the toString method is called, the person object is checked first, followed by a check on its prototype ( Object.prototype). Prototype logic is used, which typically returns .
Difference Between Objects and Arrays
Arrays have significant differences from traditional JavaScript objects. The reason lies in the Array.prototype object, which represents all the methods inherent in arrays. Each new array inherits these methods from Array.prototype .
It's important to note that the value of the prototype property in Array.prototype is Object.prototype . This means that arrays are just objects, but with additional methods. There is nothing that an object can do that an array cannot do.
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oddities
Like JavaScript objects, arrays have their own characteristics.
Non-Indexed Properties
Since arrays are just objects, you can apply non-indexed properties to them. This is usually the first thing that surprises. In the example below, I'm setting two non-indexed properties named sorted and authored by groceries array.
Note: As with objects, both dot and parenthesis are supported here.
View example
length
The length property of an array is also often confusing. This property is often confused with counting elements in an array. However, the value of length in a numeric expression is greater than the largest array index. Because of this, non-indexed properties do not affect the length of the array, as shown in the example.
Another situation in which length can be misleading is when we are trying to add an element at an index greater than the current value of the length array. Note that in the example, the length of the array jumped from 2 to 10 just after adding the third element to the array at index 9 .
When the value length properties changes, each element with an index greater than the new value of length is to be removed.
Note:
To get the correct length value, you can use Object.keys(groceries).length . Note that this also includes non-indexed properties until you define them as non-enumerable. That is:
Object.defineProperty(groceries, "sorted", ( value: false, enumerable: false, configurable: true, writable: true ));
So how to be?
If you need to create a collection of properties various types, use JavaScript object creation. In all other cases, you can use an array.
The translation of the article “JavaScript: Arrays vs Objects” was prepared by the friendly team of the project.
Arrays are a widely used data type in programming languages that is used to store multiple values at the same time. However, due to the specificity of some points JavaScript language, arrays have some functionality that is often understated. In this post, I will look at three lesser known but still important JavaScript functions arrays you might not even know existed...
Before starting, I want to note that this text is a logical continuation of a similar post - so follow the link and read 🙂
Adding Custom Properties to Arrays
If you ever searched the internet for the full definition arrays in JavaScript, then you will most likely find sources where it is written that arrays in JavaScript are objects, and as strange as it may sound, it is true.
In fact, almost everything we deal with in JavaScript is an object. Essentially there are two types data in JavaScript – primitives and objects, but primitives always wrap inside objects.
Loop through array elements
After reading this subheading, you probably thought - “Nothing new - I already know that” and you will be partially right, because anyone, even a beginner, can loop through the elements of an array, but in fact it is all quite abstract and the cycle will pass by array indexes.
Because the array indexes consist only of non-negative integers, in the loop we "iterate" the integer value, usually starting from zero and ending with a number that indicates the size of the array, and then use the "iterated" value to access the element of the array at a given index.
However, in ECMAScript6 , there is a way to iterate through the values of an array without thinking about indexes, and you can do this with a for...of loop.
In an array, the for...of loop will iterate over the elements of the array in index order, in other words, you no longer need to care about iterating over the indexes and getting the existing array values at given index. The for...of loop is perfect if you just want to loop through the elements of an array and start working with them. For example:
JavaScript
var arr = ["apple","banana","orange"]; for (let item of arr)( console.log(item); ) // "apple","banana","orange"
For comparison, let's now look at standard array loop without explicitly specifying the value index.
JavaScript
var arr = ["apple","banana","orange"]; for (var item = 0; item< arr.length; item++){ console.log(item); } // 0, 1, 2
The number of elements does not show the true size of the array
Typically, when we talk about the size of an array, we think of it as either a number having a value that the array contains, or a size that we have given the array manually. However, in reality array size depends on the largest existing index in it.
Length- (I know that this is generally called length, but I'm very used to calling it - size, so forgive me in advance if I misled anyone) - a very flexible property. Have you already fixed the size of the array beforehand or not if you continue add values to an array, its size continues to increase accordingly. For example:
JavaScript
var arr = ; arr.length = 3; console.log(arr.length); // 3 arr = "abcd"; console.log(arr.length); // 6
var arr = ; arr. length = 3 ; // 3 arr [ 5 ] = "abcd" ; console. log (arr . length ) ; // 6 |
In the example above, you can see that I gave the array only one value at index 5, and its size became 6. Now, if you think that by adding a value at index 5, the array created indices 0 to 4 automatically, then you are deeply you are wrong. There are really no existing indices from 0 to 4 in this array. You can check this using the in operator.
JavaScript
var arr = ; arr.length = 3; console.log(arr.length); // 3 arr = "abcd"; console.log(arr.length); // 6 console. log(0 in arr); // false
var arr = ; arr. length = 3 ; console. log (arr . length ) ; // 3 arr [ 5 ] = "abcd" ; console. log (arr . length ) ; |
Arrays are one of the most commonly used types of variables that allow you to store many consecutive values in “one place”. However, when it comes to JavaScript, there is room for improvement.
In this article, we will look at three little-known techniques that can be applied when working with arrays.
1. Addition custom properties to arrays
If you use search to find the definition of an array within the JavaScript language, then most sources will claim that this type of variable value is represented as an object.
Generally speaking, a lot of the things we see in JavaScript are objects. It is fair to say that there are also “primitive” data types in the language, but their values are somehow used in properties within objects.
2. Access to array elements within a loop
Since array indices can only take positive values, the counting starts from zero. Subsequently, we can use this index to access the array element at a given iteration of the loop.
ECMAScript6 introduced a way to loop through an array without using indexes, but through new cycle for…of .
The for...of loop is designed to iterate through the elements of an array without affecting the index of the element.
var ary = ["orange","apple","lychee"]; for (let item of ary)( console.log(item); ) // "orange", "apple", "lychee" For comparison: displaying item indexes in a for loop. var ary = ["orange","apple","lychee"]; for (var item = 0; item< ary.length; item++){ console.log(item); } // 0, 1, 2
3. The number of elements is not the dimension of the array
When we talk about the dimension of an array, we usually think that it means the number of elements stored in it. In fact, this is not entirely true - the length property is calculated depending on the maximum index of the element.
The length property is very ambiguous. To verify this, just look at the following manipulations:
Variary = ; ary.length = 3; console.log(ary.length); // 3 ary = "abcd"; console.log(ary.length); // 6
In the last example, it was enough to put the element in the fifth position, as a result of which the length of the array became equal to 6. If you think that indexes from 0 to 4 will be created automatically, you will be wrong. This can be checked using the in operator.
Variary = ; ary.length = 3; console.log(ary.length); // 3 ary = "abcd"; console.log(ary.length); // 6 console.log(0 in ary); // false
AT this case it is fair to call the array ary "sparse".
We can also manipulate the length property to trim arrays. The example below demonstrates the "loss" of the element at index 5 by decrementing the value of the property array length ary.
Variary = ; ary.length = 3; console.log(ary.length); // 3 ary = "abcd"; console.log(ary.length); // 6 ary.length = 2; console.log(ary.length); // 2 console.log(ary); // undefined
JavaScript is designed around a simple paradigm. The concept is based on simple objects. An object is a set of properties, and each property consists of a name and a value associated with that name. The property value can be a function that can be called method object. In addition to the browser's built-in objects, you can define your own objects. This chapter describes how to use objects, properties, functions, and methods, as well as how to create your own objects.
Object Overview
Objects in JavaScript, like in many other programming languages, are similar to objects. real life. concept JavaScript objects it is easier to understand by drawing parallels with objects that really exist in life.
In JavaScript, an object is a self-contained entity that has properties and a specific type. Compare, for example, with a cup. A cup has a color, shape, weight, material from which it is made, and so on. Similarly, JavaScript objects have properties that define their characteristics.
Objects and Properties
In JavaScript, an object has properties associated with it. A property of an object can be understood as a variable assigned to an object. Object properties are essentially the same JavaScript variables, except they are attached to the object. The properties of an object determine its characteristics. You can access an object property using dot notation:
ObjectName.propertyName
Like all JavaScript variables, object name (which can also be a variable), and property name are case-sensitive. You can define a property by specifying its value. For example, let's create a myCar object and define its make , model , and year properties as follows:
Var myCar = new Object(); myCar.make = "Ford"; myCar.model = "Mustang"; myCar.year = 1969;
Undefined object properties are undefined (not null).
mycar. color; // undefined
Properties of JavaScript objects can also be accessed or set using bracket notation (see ). Objects are sometimes called associative arrays , since each property is associated with a string value that can be used to access it. So, for example, you can access the properties of the myCar object like this:
MyCar["make"] = "Ford"; myCar["model"] = "Mustang"; myCar["year"] = 1969;
Object property names can be JavaScript strings, or anything that can be converted to a string, including the empty string. However, any property name that contains an invalid JavaScript identifier (for example, a property name containing a space and a dash or starting with a number) can be accessed using square brackets. This notation is also useful when property names must be dynamically defined (when the property name is not defined until runtime). Examples below:
Var myObj = new Object(), str = "myString", rand = Math.random(), obj = new Object(); myObj.type = "dot syntax"; myObj["date created"] = "String with space"; myObj = "string value"; myObj = "Random Number"; myObj = "Object"; myObj[""] = "Even an empty string"; console log(myObj);
Note that all keys with square brackets are converted to type String because objects in JavaScript can only have String as a key. For example, in the code above, when the key obj is added to myObj , JavaScript calls the obj.toString() method and uses that resulting string as the new key.
You can also access properties using the string value stored in a variable:
var propertyName = "make"; myCar = "Ford"; propertyName = "model"; myCar = "Mustang";
You can use square brackets in a for...in construct to iterate through all the properties of an object for which it is allowed. To show how this works, the following function shows all the properties of an object when you pass the object itself and its name as arguments to the function:
Function showProps(obj, objName) ( var result = ""; for (var i in obj) ( if (obj.hasOwnProperty(i)) ( result += objName + "." + i + " = " + obj[i ] + "\n"; ) ) return result; )
So if we call this function like this showProps(myCar, "myCar"), we get the result:
myCar.make=Ford myCar.model=Mustang myCar.year=1969
Listing all object properties
Using a constructor function
Another way to create an object in two steps is described below:
- Determine the type of an object by writing a constructor function. The name of such a function, as a rule, begins with a capital letter.
- Instantiate an object using the new keyword.
To determine the type of an object, create a function that determines the type of the object, its name, properties, and methods. For example, suppose you want to create an object type to describe machines. You want this type of object to be called car , and you want it to have make , model , and year properties. To do this, write the following function:
Function Car(make, model, year) ( this.make = make; this.model = model; this.year = year; )
Note that this is used to assign values (passed as function arguments) to the object's properties.
Now you can create an object called mycar like this:
Var mycar = new Car("Eagle", "Talon TSi", 1993);
This statement creates an object of type Car with reference mycar and assigns certain values to its properties. The value of mycar.make will be the string "Eagle", mycar.year will be the integer 1993, and so on.
You can create as many car objects as you like by simply calling new . For example:
Varkenscar = new Car("Nissan", "300ZX", 1992); var vpgscar = new Car("Mazda", "Miata", 1990);
An object can have a property that is another object. For example, the following defines an object of type Person as follows:
Function Person(name, age, sex) ( this.name = name; this.age = age; this.sex = sex; )
and then create two new instances of Person objects as follows:
Var rand = new Person("Rand McKinnon", 33, "M"); var ken = new Person("Ken Jones", 39, "M");
Then, you can rewrite the car definition to include an owner property that you can assign a person object to, like so:
Function Car(make, model, year, owner) ( this.make = make; this.model = model; this.year = year; this.owner = owner; )
Then, to instantiate new objects, run the following instructions:
Var car1 = new Car("Eagle", "Talon TSi", 1993, rand); var car2 = new Car("Nissan", "300ZX", 1992, ken);
Note that instead of passing a string, literal, or integer when creating new objects, the expressions above pass the rand and ken objects as an argument to the function. Now, if you need to know the name of the owner of car2, you can do it like this:
Car2.owner
Note that at any time you can add a new property to a previously created object. For example, the expression
car1.color = "black";
adds a color property to car1, and sets its value to "black." However, it does not affect any other objects. To add a new property to all objects of the same type, you must add the property to the type definition of the car object.
Using the Object.create Method
Objects can also be created using the Object.create method. This method is very handy as it allows you to specify a prototype object for your new object without defining a constructor function.
// list of properties and methods for Animal var Animal = ( type: "Invertebrates", // Default value of type displayType: function() ( // Method displaying the type of Animal object console.log(this.type); ) ); // Create an Animal object var animal1 = Object.create(Animal); animal1.displayType(); // Output: Invertebrates // Create an Animal object and assign it type = Fishes var fish = Object.create(Animal); fish.type = "Fishes"; fish.displayType(); // Output:Fishes
Inheritance
All objects in JavaScript inherit from at least another object. The object that is inherited from is called the prototype, and the inherited properties can be found in the constructor's prototype object.
Object Property Indices
In JavaScript 1.0, you can refer to the properties of an object either by its name or by its ordinal index. In JavaScript 1.1 and later, if you originally defined a property by name, you must always refer to it by its name, and if you originally defined a property by index, you must refer to it by its index.
This restriction is imposed when you create an object and its properties using a constructor function (as we did earlier with the type car) and when you define individual properties explicitly (eg myCar.color = "red"). If you originally defined a property of an object via an index, such as myCar = "25 mpg" , then later on you can refer to this property only as myCar .
The exception to the rule is objects rendered from HTML, such as the forms array. You can always refer to the objects in these arrays either by using their index (which is based on the order of appearance in HTML Document), or by their names (if any have been defined). For example, if the second html tag