VSCode Formatter Extension Example

formatter.js

var vscode = require("vscode")

function activate(context) {
    console.log('Formatter activated') // formatting only works once activated
    const formatter = vscode.languages.registerDocumentFormattingEditProvider(
        'language-id', // set this to the language ID of the language to format (the bit in brackets in the vscode language menu)
        {
            provideDocumentFormattingEdits: function (document) {

                const output = document.getText()
                    //add your formatting replacements here
                    .replace('text!', 'replacement1')
                    .replace(/regex/, 'replacement2')

                // condense the following lines if you like
                const startOfDocument = document.lineAt(0).range.start
                const endOfDocument = document.lineAt(document.lineCount-1).range.end
                const documentRange = new vscode.Range(startOfDocument, endOfDocument)
                const replacedFileContent = vscode.TextEdit.replace(documentRange, output)

                return [replacedFileContent]

            }
        }
    );
    context.subscriptions.push(formatter) // let vscode know about the formatter
}

exports.__esModule = true
exports.activate = activate

package.jsonc

// actual filename package.json
{
    //: ...
    "main": "./formatter", //formatter.js
    "activationEvents": [
        "onLanguage:language-id" // activate extention for files set to this language
    ],
    // ...
}