Flattening function for arbitrarily nested arrays in TypeScript 2.1

flattening.ts

// Note: Prefer static typing over defensive programming, lots of tests
// Note: The simple recursive impl. here is sufficient when dealing with small input (e.g unwrapping JSON), for potentially large input the recursion should be unrolled

interface Nested<T> extends Array<T | Nested<T>> { }

function* flatten<T>(items: Nested<T>): Iterable<T> {
  for (var i of items)
    if (Array.isArray(i)) yield* flatten(i);
    else yield i;
}

// Tests
console.log(Array.from(flatten([])));                         // []                           Empty array
console.log(Array.from(flatten([[1,2,[3, [1, 2, 1, 3]]],4]))) // [ 1, 2, 3, 1, 2, 1, 3, 4 ]   Sample input
console.log(Array.from(flatten(["hello", ["world"]])));       // [ "hello", "world" ]         Array of iterables

// Runtime errors
const a: any[] = [];
a.push(a);
Array.from(flatten(a)); // Stack overflow

// Compile errors
flatten();     // empty
flatten(null); // null with --strictNullChecks enabled
flatten(10);   // non-array
flatten("");   // other iterable

tsconfig.json

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es2015",
        "noImplicitAny": true,
        "sourceMap": false,
        "strictNullChecks": true
    },
    "files": [
        "main.ts"
    ]
}