Features

Syntax Highlighting for ActionScript, MXML, and JSFL files.
IntelliSense completion for classes and interfaces, imports, properties, methods, and more.
Signature Help shows a list of a function’s parameters when you call it.
Errors and Warnings are updated in real time as you type (or on save only, if you prefer).
Hover over a symbol to see more details such as types, namespaces, documentation, and more.
Go to Definition with Ctrl+Click on any usage of a symbol to open the file where it is defined. Can be used to see the public API of classes from compiled .swc libraries too.
Find All References for any symbol in the project across all source files.
Rename Symbol for classes, interfaces, methods, and variables.
Organize Imports sorts imports alphabetically, removes unused imports, and adds missing imports.
Quick Fixes to add missing imports, generate missing variables or methods, or convert variables to getters/setters.
Outline view lists all symbols from the current file in a hierarchical tree.
Go to Symbol in Workspace with Ctrl+T and type the name of any symbol to search for it in the workspace.
Build Tasks can compile a project or package an Adobe AIR app with Ctrl+Shift+B.
Adobe Animate integration includes Test Movie, Debug Movie, and Publish.
Import Projects from Adobe Flash Builder or FlashDevelop.

Download Visual Studio Code

https://code.visualstudio.com/Download

Install extension

Search for AS3 and install AS3, MXML, and SWF Extension Pack.

https://marketplace.visualstudio.com/items?itemName=bowlerhatllc.vscode-nextgenas

Download AIR SDK

Download https://airsdk.harman.com/download and put somewhere inside a new folder.

Select AIR SDK

Now with your project folder open

Check the bottom right and click No SDK

Or press F1 and search:

ActionScript SDK

Now select the folder with the AIR SDK

Making the config

Make a new file with the name:

asconfig.json

with

{
	"compilerOptions": {
		"source-path": [
			"/"
		],
		"output": "bin/Main.swf"
	},
	"mainClass": "Main",
	"animateOptions": {
		"file": "Main.fla"
	}
}

For more config check https://github.com/BowlerHatLLC/vscode-as3mxml/wiki/asconfig.json