What is use of $ in javascript?

The most common operators are the arithmetic, logical, and comparison operators. But did you know that JavaScript has an in operator?

If you didn't, don’t fret. I just came across it recently while searching for a solution to a problem on Google.

In this article, you’ll learn exactly what the JavaScript in operator does, when to use it, and how to use it.

The JavaScript in operator is used to check if a specified property exists in an object or in its inherited properties (in other words, its prototype chain). The in operator returns true if the specified property exists.

The JavaScript prototype chain is how objects or object instances have access to properties and methods that were not originally theirs. These objects inherit properties and methods defined in their constructors or prototypes, which can be accessed through their __proto__ property.

This article assumes that you have a basic understanding of what objects are, how to create them, what they are used for, and how JavaScript inheritance works. If you don’t, this article on MDN should help.

Let’s use the ES6 class syntax to create an object constructor. This would also apply to function constructors:

You might be wondering, since we established that the JavaScript in operator can be used with objects, why can we also use it with arrays?

Well, an array is actually a prototype (instance) of the Object type. In fact, everything in JavaScript is an instance of the Object type.

That may sound crazy, but lets run a simple program in the browser's console to confirm.

First, define an array and confirm if its an instance of the Object type using the instanceof operator:

Still in doubt? Type number into the console and press enter, then open up the output.

You’ll notice a list of properties, one of which is __proto__ which points to Array. Opening that too and going down that list bring us to another __proto__ property with a value of Object.

That shows that the number array is an instance of the Array type which is an instance of the Object type.

Now, back to using the in operator:

In Kirupa's article, Check If You Are On a Touch Enabled Device, he highlights this function:

This function returns true if you are on a device that supports touch and returns false if you are on a device that doesn't support touch by checking if the properties window.navigator.msMaxTouchPoints and ontouchstart are present. These properties only exist on devices that are touch enabled.

Pretty straightforward!

Lets focus on the highlighted line. Remember how we established that the in operator returns true if the specified property exists in an object? HTML elements used in JavaScript actually become instances of the Object type, hence the name "Document Object Model" or DOM.

Of course, you might not believe me without some sort of proof. As before, let’s type some commands into the console.

Create a div element and list out its properties using console.dir():

You'll then see the div element with its properties listed in the console.

Open the drop down and you’ll notice that it has a __proto__ property of HtmlDivElement. Open that and you’ll find another __proto__ property of HtmlElement, then Element, Node, Eventtarget, and finally Object.

Also run:

This will return true, showing that the div element is an instance of the Object type, which is why the in operator can be used on it.

You’ve learned about the not so popular JavaScript in operator, which is used to verify the presence of properties on an object or Object type instances. This should come in handy when writing verification logic.

For this reason, these characters are not treated the same way as other special symbols. Instead, JavaScript treats $ and _ as if they were letters of the alphabet.

A JavaScript identifier — again, just a name for any object — must start with a lower or upper case letter, underscore (_), or dollar sign ($); subsequent characters can also include digits (0-9). Anywhere that an alphabetic character is allowed in JavaScript, 54 possible letters are available: any lowercase letter (a through z), any uppercase letter (A through Z), $ and _.

The dollar sign is commonly used as a shortcut to the function document.getElementById(). Because this function is fairly verbose and used frequently in JavaScript, the $ has long been used as its alias, and many of the libraries available for use with JavaScript create a $() function that references an element from the DOM if you pass it the id of that element.

There is nothing about $ that requires it to be used this way, however. But it has been the convention, although there is nothing in the language to enforce it.

The dollar sign $ was chosen for the function name by the first of these libraries because it is a short one-character word, and $ was least likely to be used by itself as a function name and therefore the least likely to clash with other code in the page.

Now multiple libraries are providing their own version of the $() function, so many now provide the option to turn off that definition in order to avoid clashes.

Of course, you don't need to use a library to be able to use $(). All you need to substitute $() for document.getElementById() is to add a definition of the $() function to your code as follows:

A convention has also developed regarding the use of _, which is frequently used to preface the name of an object's property or method that is private. This is a quick and easy way to immediately identify a private class member, and it is so widely used, that almost every programmer will recognize it.

This is particularly useful in JavaScript since defining fields as private or public is done without the use of the private and public keywords (at least this is true in the versions of JavaScript used in web browsers — JavaScript 2.0 does allow these keywords).

Note that again, as with $, the use of _ is merely a convention and is not enforced by JavaScript itself. As far as JavaScript is concerned, $ and _ are just ordinary letters of the alphabet.

Syntax:

Parameters: This function accepts the following parameter as mentioned above and described below:

Return value: This method returns a boolean value:

Example 1: Below is an example of the in-operator.

Output:

Example 2: This example shows the use of the in operator in Javascript.

Output:

Example 3: This example shows the use of the in operator in Javascript.

Output:

We have a complete list of Javascript Operators, to check those please go through the Javascript Operators Complete Reference article.

Supported Browsers: The browsers supported by JavaScript in operator are listed below:

Arithmetic operators are used to perform arithmetic between variables and/or values.

Given that y = 5, the table below explains the arithmetic operators:

For a tutorial about arithmetic operators, read our JavaScript Arithmetic Tutorial.

Assignment operators are used to assign values to JavaScript variables.

Given that x = 10 and y = 5, the table below explains the assignment operators:

For a tutorial about assignment operators, read our JavaScript Assignment Tutorial.

The + operator, and the += operator can also be used to concatenate (add) strings.

Given that t1 = "Good ", t2 = "Morning", and t3 = "", the table below explains the operators:

Comparison operators are used in logical statements to determine equality or difference between variables or values.

Given that x = 5, the table below explains the comparison operators:

For a tutorial about comparison operators, read our JavaScript Comparisons Tutorial.

The conditional operator assigns a value to a variable based on a condition.

Logical operators are used to determine the logic between variables or values.

Given that x = 6 and y = 3, the table below explains the logical operators:

The ?? operator returns the first argument if it is not nullish (null or undefined).

Otherwise it returns the second argument.

The nullish operator is supported in all browsers since March 2020:

The ?. operator returns undefined if an object is undefined or null (instead of throwing an error).

The optional chaining operator is supported in all browsers since March 2020:

Bit operators work on 32 bits numbers. Any numeric operand in the operation is converted into a 32 bit number. The result is converted back to a JavaScript number.

The typeof operator returns the type of a variable, object, function or expression:

Please observe:

The delete operator deletes a property from an object:

The delete operator deletes both the value of the property and the property itself.

After deletion, the property cannot be used before it is added back again.

The delete operator is designed to be used on object properties. It has no effect on variables or functions.

The ... operator expands an iterable into more elements:

The ... operator can be used to expand an iterable into more arguments for function calls:

The in operator returns true if a property is in an object, otherwise false:

The instanceof operator returns true if an object is an instance of a specified object:

The void operator evaluates an expression and returns undefined. This operator is often used to obtain the undefined primitive value, using "void(0)" (useful when evaluating an expression without using the return value).

For this reason, these characters are not treated the same way as other special symbols. Instead, JavaScript treats $ and _ as if they were letters of the alphabet.

A JavaScript identifier — again, just a name for any object — must start with a lower or upper case letter, underscore (_), or dollar sign ($); subsequent characters can also include digits (0-9). Anywhere that an alphabetic character is allowed in JavaScript, 54 possible letters are available: any lowercase letter (a through z), any uppercase letter (A through Z), $ and _.

The dollar sign is commonly used as a shortcut to the function document.getElementById(). Because this function is fairly verbose and used frequently in JavaScript, the $ has long been used as its alias, and many of the libraries available for use with JavaScript create a $() function that references an element from the DOM if you pass it the id of that element.

There is nothing about $ that requires it to be used this way, however. But it has been the convention, although there is nothing in the language to enforce it.

The dollar sign $ was chosen for the function name by the first of these libraries because it is a short one-character word, and $ was least likely to be used by itself as a function name and therefore the least likely to clash with other code in the page.

Now multiple libraries are providing their own version of the $() function, so many now provide the option to turn off that definition in order to avoid clashes.

Of course, you don't need to use a library to be able to use $(). All you need to substitute $() for document.getElementById() is to add a definition of the $() function to your code as follows:

A convention has also developed regarding the use of _, which is frequently used to preface the name of an object's property or method that is private. This is a quick and easy way to immediately identify a private class member, and it is so widely used, that almost every programmer will recognize it.

This is particularly useful in JavaScript since defining fields as private or public is done without the use of the private and public keywords (at least this is true in the versions of JavaScript used in web browsers — JavaScript 2.0 does allow these keywords).

Note that again, as with $, the use of _ is merely a convention and is not enforced by JavaScript itself. As far as JavaScript is concerned, $ and _ are just ordinary letters of the alphabet.