extension.js

// The module 'vscode' contains the VS Code extensibility API
// Import the module and reference it with the alias vscode in your code below
const vscode = require('vscode');

// this method is called when your extension is activated
// your extension is activated the very first time the command is executed

/**
 * @param {vscode.ExtensionContext} context
 */
function activate(context) {

    let disposable = vscode.commands.registerCommand('dumb-xml-prettifier.prettify', function() {

        var editor = vscode.window.activeTextEditor;

        var selectedText = editor.document.getText(editor.selection);

        var pretty = prettifier();

        if (selectedText) {

            var returnText = pretty.prettifyXml(selectedText);

            editor.edit(editBuilder => {
                editBuilder.replace(editor.selection, returnText);
            });
        } else {

            var docText = editor.document.getText();

            var returnText = pretty.prettifyXml(docText);

            var lineCount = vscode.window.activeTextEditor.document.lineCount - 1;
            var charCount = editor.document.lineAt(editor.document.lineCount - 1).text.length;

            // Overwrite text in active document.
            editor.edit(builder => {
                builder.delete(new vscode.Range(new vscode.Position(0, 0), new vscode.Position(lineCount, charCount)));
                builder.insert(new vscode.Position(0, 0), returnText);
            });
        }
    });

    context.subscriptions.push(disposable);
}

// this method is called when your extension is deactivated
function deactivate() { }

module.exports = {
    activate,
    deactivate
}

var prettifier = function() {

    let outputXmlLines = [];
    let paddingLevel = 0;

    function header(xmlLines, index) {

        paddingLevel--;

        outputXmlLines.push((index == 0 ? "" : "\n") + xmlLines[index]);
    }

    function openingTag(line, previousLine) {

        if (previousLine != undefined) {
            // /> at the end of the line
            if (!previousLine.match(/\/>$/)) {

                var onlyElementName = line.match(/^<(.+?)(\s|>$)/)[1];

                var pattern = "^</" + onlyElementName + ">";

                if (!previousLine.match(pattern)) {
                    paddingLevel++;
                }
            } else {
                paddingLevel++
            }
        } // Padding stays the same because its the first line.

        let padding = "";
        let paddingAmount = paddingLevel * 4;

        for (let i = 0; i < paddingAmount; i++) {
            padding += " ";
        }

        line = line.replace(/(.)\s\/>$/, "$1/>") // remove any space before closing tag />

        var newStrArr = indentAttributes(line, paddingAmount, padding).flatMap((a) => a);

        // Cancel out closing tag />
        if (line.match(/\/>$/)) {
            paddingLevel--;
        }

        outputXmlLines.push(newStrArr.join(""));
    }

    function indentAttributes(line, paddingAmount, existingPadding) {

        var elementParts = line
            .split(/([a-zA-Z-:_]+="[\w\d\s\-\.\*\+\=]+"\/?>?)/g)
            .filter(f => f != "")
            .filter(f => f != " ")
            .map(m => m.trim());

        return elementParts.map(f => {

            var amountOfLocalPadding = paddingAmount + 4;

            let localPadding = "";

            for (let i = 0; i < amountOfLocalPadding; i++) {
                localPadding += " ";
            }

            return (f.substring(0, 1) == "<" ? "" + existingPadding : "\n" + localPadding) + f;
        });
    }

    function closingTag(xmlLines, index) {

        if (index != 0) {
            // Check that count wasn't already decreased due to previous tag ending with />
            if (!xmlLines[index - 1].match(/\/>$/)) {

                var openingTag = xmlLines[index].replace('/', '').replace('>', '');

                if (!xmlLines[index - 1].match(openingTag)) {
                    paddingLevel--;
                }
            }
        } else {
            paddingLevel--;
        }

        let padding = "";
        var paddingAmount = paddingLevel * 4;

        for (let i = 0; i < paddingAmount; i++) {
            padding += " ";
        }

        outputXmlLines.push(padding + xmlLines[index]);
    }

    return {
        prettifyXml: function(xml) {

            let xmlLines = xml
                .split(/(<[^>]*>)/gm)
                .filter(m => m !== "")
                .map(m => m.trim())
                .filter(f => f != '')
                .map(f => f.replace(/\r\n/g, " "));

            // Reset state
            paddingLevel = 0;
            outputXmlLines = [];

            for (let index = 0; index < xmlLines.length; index++) {

                var currentLine = xmlLines[index];

                // Header thing, <?xml version="1.0" encoding="UTF-8"?>
                if (currentLine.match(/^<\?/)) {
                    header(xmlLines, index);
                    continue;
                }

                // Comment, <!-- like this -->
                if (currentLine.match(/^<!/)) {
                    outputXmlLines.push(currentLine);
                    continue;
                }

                // Opending tag
                if (currentLine.match(/^<[^!?\/]/)) {
                    openingTag(currentLine, xmlLines[index - 1]);
                    continue;
                }

                // Closing tag
                if (currentLine.match(/^<\//)) {
                    closingTag(xmlLines, index);
                    continue;
                }
            }

            return outputXmlLines.join("\n").replace(/\n\s+\n/g, '\n');
        }
    }
}