How to Run JavaScript Code
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sanya sanya
The primary way of writing and executing the JavaScript code is to write it in the "Script" of the HTML Document.
The example of the JavaScript written in the HTML Document using the Script tag is mentioned below -
<body>
<p>This is the line before adding JavaScript</p>
<script>
console.log( 'Hello world!' );
</script>
<p>This is the line after adding javascript...</p>
</body>
</html>
External Scripts
There is an alternate method of writing and executing JavaScript in the HTML Document. The method is for inserting external scripts with the script tag using the src attribute.// < script src="/js/script1.js">
Scripts: Async and Defer
Most importantly in modern JavaScript applications or websites, the scripts sometimes got heavier than the HTML Document which makes the performance down and downloading time higher.When the HTML Document gets loaded by the Browser and the script src tag, it can go on with the DOM Building and the script must be loaded as soon it gets countered in the HTML Document execution. The same goes with the Javascript in the script tag without the "src" attribute. The browser runs this and then the further execution of the HTML Document proceeds.
In such case Script tag is like -
However, this leads to two problems to rise and both are mentioned below -
1- Script add handlers because they can't see the DOM Elements below them.
2- The user cannot see the content of the page until and unless the script gets loaded.
The solution for that is the Defer.
Defer
The Defer is an attribute that tells the browser not to wait for the script tag to be loaded. However, the browser will continue to execute the HTML Document. The code for the same is mentioned below -<p>...content before script...</p>
<script defer src="main.js"></script>
<!-- visible immediately -->
<p>...content after script...</p>
Defer is like not waiting for the script to be loaded -
Async
Async is an attribute that let the javascript code inside the script tag load"asynchronously". If the script is async it means that it will load independently of the other scripts.
Both the attributes "defer" and "async" are somewhat the same. Defer also make the script "non-blocking".
Comments in Javascript
Comments in javascript are the lines of code or simple language written to explain the code which doesn't affect the execution of the code. There are two types of comments supported by javascript.
Single Line Comment
These comments are single-liner which indicates the code or message which will not affect the execution of the code. It starts with the sign "//" and the example for the same is as follows -
//program to add two numbers let a = 5; let b = 2; console.log(a + b);
Output: 7
Multiline Comment
Multiline Comments have the same functionality and the only difference is that it is used to comment more than one line at once. It is started with "/" and ends at "/". An example of the same is mentioned below -
/* let a = 5; let b = 2; console.log(a + b); */
The above code will not provide any output because the whole program is written under comment. Hence, the program will not execute.
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An Introduction to Javascript!
How to Run JavaScript Code
Variables in Javascript
Numbers in JavaScript
JavaScript Operators
Data Types in JavaScript
Conditional Statements
Switch Statements
Loops in Javascript
Arrays in JavaScript
Strings in JavaScript
Objects in JavaScript
Object Methods in JavaScript
Functions in JavaScript
Object Referencing and Copying in JavaScript
' this' keyword
Asynchronous Programming in JavaScript
Callbacks in JavaScript
Promises in JavaScript
Constructor Functions in JavaScript
Async and Await in JavaScript
Type Conversion in Javascript
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Currying in JavaScript
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Comments in Javascript
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