var noop = function() {}; var identity = function(foo) { return foo; }; function println(value: string); function getArray(): number[] { return [1, 2]; } type Foo = { functionInFoo(): void; }; const good = () => 0; const good = (x) => 0; const bad = (a => [...a, b]); const bad = (_ => doSomething()); const bad = (() => 0); const bad = ((a, b) => [...a, b]); const array = [1, 2, 3].reduce<number[]>((acc, next) => [...acc, next], []); const bad = ((a=2, b=5) => [...a, b]); sides.every((length,width=(3+2+(4/5))) => length > 0 );