How to simulate let expressions in JavaScript?

Question

Consider the following implementation of take:

const take = (n, [x, ...xs]) =>
    n === 0 || x === undefined ?
    [] : [x, ...take(n - 1, xs)];

console.log(take(7, [1, 2, 3, 4, 5])); // [1, 2, 3, 4, 5]
console.log(take(3, [1, 2, 3, 4, 5])); // [1, 2, 3]
console.log(take(1, [undefined, 1]));  // []

As you can see, it doesn't work for arrays with undefined because x === undefined is not the best way to test whether an array is empty. The following code fixes this problem:

const take = (n, xs) =>
    n === 0 || xs.length === 0 ?
    [] : [xs[0], ...take(n - 1, xs.slice(1))];

console.log(take(7, [1, 2, 3, 4, 5])); // [1, 2, 3, 4, 5]
console.log(take(3, [1, 2, 3, 4, 5])); // [1, 2, 3]
console.log(take(1, [undefined, 1]));  // [undefined]

However, writing xs[0] and xs.slice(1) isn't as elegant. In addition, it's problematic if you need to use them multiple times. Either you'll have to duplicate code and do unnecessary extra work, or you'll have to create a block scope, define constants and use the return keyword.

The best solution would be to use a let expression. Unfortunately, JavaScript doesn't have them. So, how to simulate let expressions in JavaScript?

Answer 1

In Lisp, a let expression is just syntactic sugar for a left-left lambda (i.e. an immediately-invoked function expression). For example, consider:

(let ([x 1]
      [y 2])
  (+ x y))

; This is syntactic sugar for:

((lambda (x y)
    (+ x y))
  1 2)

In ES6, we can use arrow functions and default parameters to create an IIFE that looks like a let expression as follows:

const z = ((x = 1, y = 2) => x + y)();

console.log(z);

Using this hack, we can define take as follows:

const take = (n, xxs) =>
    n === 0 || xxs.length === 0 ?
    [] : (([x, ...xs] = xxs) => [x, ...take(n - 1, xs)])();

console.log(take(7, [1, 2, 3, 4, 5])); // [1, 2, 3, 4, 5]
console.log(take(3, [1, 2, 3, 4, 5])); // [1, 2, 3]
console.log(take(1, [undefined, 1]));  // [undefined]

However, it would be best to use statements instead of expressions here.

const take = (n, xxs) => {
  if (n === 0 || xxs.length === 0) return [];
  const [x, ...xs] = xxs;
  return [x, ...take(n - 1, xs)];
};

console.log(take(7, [1, 2, 3, 4, 5])); // [1, 2, 3, 4, 5]
console.log(take(3, [1, 2, 3, 4, 5])); // [1, 2, 3]
console.log(take(1, [undefined, 1]));  // [undefined]