extension.ts

import * as vscode from 'vscode';

export function activate(context: vscode.ExtensionContext) {
    context.subscriptions.push(vscode.commands.registerCommand('jrieken.backspaceLeft', backspace));
}


function backspace() {

    if (vscode.window.activeTextEditor) {

        const { document, selections, options: { tabSize } } = vscode.window.activeTextEditor;

        if (typeof tabSize === 'number') {

            let hasNewSelections = false;
            const newSelections = selections.map(selection => {
                if (!selection.isEmpty) {
                    return selection;
                }

                const { start } = selection; // since it's empty start is the same as 'active'
                if (start.character === 0) {
                    return selection;
                }

                const line = document.lineAt(start);
                if (line.firstNonWhitespaceCharacterIndex < start.character) {
                    return selection;
                }

                // check for n-space characters preceeding the caret
                let match = /^\t*((?: )+)$/.exec(line.text.substr(0, start.character));
                if (match) {
                    hasNewSelections = true;
                    let toRemove = (match[1].length % tabSize) || tabSize;
                    return new vscode.Selection(start.with(void 0, start.character - toRemove), start);
                }

                return selection;
            });

            // set the new (expanded) selections and run 'deleteLeft'
            if (hasNewSelections) {
                vscode.window.activeTextEditor.selections = newSelections;
            }
        }
    }

    return vscode.commands.executeCommand('deleteLeft');
}