Configuration

import ts from "typescript";

function createPlugin(info: ts.server.PluginCreateInfo): ts.LanguageService {
  const ls = info.languageService;
  const prefix: string = info.config?.prefix ?? "[lint]";
  return {
    ...ls,
    getSemanticDiagnostics: function(fileName: string) {
      const prior = ls.getSemanticDiagnostics(fileName);
      const sf = ls.getProgram().getSourceFile(fileName);
      if (!sf) return prior;
      const extras: ts.Diagnostic[] = [];
      function visit(node: ts.Node) {
        if (ts.isVariableDeclaration(node) && !node.type) {
          extras.push({
            file: sf,
            start: node.getStart(sf),
            length: node.getWidth(sf),
            category: ts.DiagnosticCategory.Warning,
            code: 9001,
            messageText: prefix + " Missing type annotation",
          });
        }
        ts.forEachChild(node, visit);
      }
      visit(sf);
      return [...prior, ...extras];
    },
  };
}

const sf = ts.createSourceFile("app.ts", "let x = 1;", ts.ScriptTarget.Latest, true);
const fakeLs = {
  getSemanticDiagnostics: function() { return [] as ts.Diagnostic[]; },
  getProgram: function() { return { getSourceFile: function(n: string) { return n === "app.ts" ? sf : undefined; } } as unknown as ts.Program; },
} as unknown as ts.LanguageService;

const info = { languageService: fakeLs, config: { prefix: "[X]" } } as unknown as ts.server.PluginCreateInfo;
const plugin = createPlugin(info);
const diags = plugin.getSemanticDiagnostics("app.ts");
console.log((diags[0] as ts.Diagnostic).messageText);