Four ways to define functions in TypeScript

fun.spec.ts

import "jest";

// C-style declared function with block body
function increment_block(i: number): number {
  return i + 1;
}

// original JavaScript inline function
const increment_var_keyword = function(i: number): number {
  return i + 1;
};

// fat-arrow inline function with block body
const increment_var_block = (i: number): number => {
  const j = i + 1;
  return j;
};

// fat-arrow inline function with expression body
const increment_var = (i: number): number => i + 1;

// The two fat-arrow variants can unfortunately collide when one wants to
// return an object literal. In that case, we need a pair of parenthesis to
// disambiguate.
const makeObject = () => ({ a: 15, b: 16 });

test("functions", () => {
    expect(increment_block(1)).toBe(2);
    expect(increment_var_keyword(1)).toBe(2);
    expect(increment_var_block(1)).toBe(2);
    expect(increment_var(1)).toBe(2);
});