How to fix Module has no exported member 'default' in TypeScript

// If module has named exports like this:
export function calculate() { /* ... */ }
export const PI = 3.14;
export class Calculator { /* ... */ }

// You CANNOT import it like this:
import MyModule from "./calculator";  // ERROR: No default export

// Instead use named imports:
import { calculate, PI, Calculator } from "./calculator";

// Module: math-utils.ts
export function add(a: number, b: number) { return a + b; }
export function multiply(a: number, b: number) { return a * b; }

// WRONG - trying to import default
import mathUtils from "./math-utils";  // ERROR

// CORRECT - import named exports
import { add, multiply } from "./math-utils";

// CORRECT - import everything as namespace
import * as mathUtils from "./math-utils";
mathUtils.add(1, 2);

{
  "compilerOptions": {
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true
  }
}

// With esModuleInterop: true
import express from "express";  // Works even though express uses module.exports
// Compiled to: const express = require("express").default || require("express")

// Without esModuleInterop: true
import * as express from "express";  // Must use namespace import
// or
import express = require("express");  // TypeScript-specific syntax

// package.json indicating ES modules
{
  "type": "module",
  "exports": {
    ".": {
      "import": "./dist/index.mjs",
      "require": "./dist/index.cjs"
    }
  }
}

// package.json indicating CommonJS (default)
{
  "main": "./dist/index.js"
}

{
  "compilerOptions": {
    "module": "node16",
    "moduleResolution": "node16"
  }
}

// Pattern 1: Namespace import (works with esModuleInterop: false)
import * as fs from "fs";
fs.readFileSync("./file.txt");

// Pattern 2: Require syntax (TypeScript-specific)
import fs = require("fs");

// Pattern 3: Destructured require (for specific exports)
const { readFileSync } = require("fs");

// Pattern 4: With esModuleInterop: true, you can often use default import
import fs from "fs";  // Only works with esModuleInterop: true

// For modules that export a single function/class:
// If module has: module.exports = MyClass
import MyClass from "./MyClass";  // With esModuleInterop: true

// types/my-module.d.ts
declare module "my-module" {
  // If module has default export
  export default function myFunction(): void;

  // If module has named exports
  export function helper(): string;
  export const version: string;

  // If module uses CommonJS export
  export = myFunction;
  // or for object exports
  namespace myModule {
    export function doSomething(): void;
  }
  export = myModule;
}