SICP Goodness - A guess of why was the name 'let' chosen for block-scoped variable declarations in JavaScript

A guess about the origin of the new let keywork in javascript

3 minute read

Do you think Computer Science equals building websites and mobile apps?

Are you feeling that you are doing repetitive and not so intelligent work?

Are you feeling a bit sick about reading manuals and copy-pasting code and keep poking around until it works all day long?

Do you want to understand the soul of Computer Science?

If yes, read SICP!!!

In this post I give my guess of where the new let keyword in JavaScript comes from.

I got this idea from (of course) reading one chapter of the SICP book: Using let to create local variables.

Suppose we want to compute the function:

f(x, y) = x(1 + xy)^2 + y(1 - y) + (1 + xy)(1 - y)

which we could also express as

a = 1 + xy

b = 1 - y

f(x, y) = xa^2 + yb + ab

How do we do this? You may think of assignment first, but remember the book hasn’t introduced the concept of assignment yet.

OK, turns out whenever you get stuck in functional programming languages, the anwser is almost always lambda!

(define (f x y)
  ((lambda (a b)
    (+ (* x (square a))
       (* y b)
       (* a b)))
   (+ 1 (* x y))
   (- 1 y)))

This is not hard to read, basically we just created a lambda procedure with parameters a and b, and then immediately call it with the right parameters (reminds you of IIFE in JavaScript?).

So a new lambda function is the way to create local variables in Scheme.

This construct is so useful that there is a special form called let to make its use more convenient.

(define (f x y)
  (let ((a (+ 1 (* x y)))
        (b (- 1 y)))
    (+ (* x (square a))
       (* y b)
       (* a b))))

The way this happens is that the let expression is interpreted as an alternate syntax for the previous lambda way.

Let’s now take a look at the JavaScript side.

Before ES6, in order for this to work as expected, we need to wrap i into a function call.

var callbacks = [];
for (var i = 0; i <= 2; i++) {
    (function (i) {
        callbacks[i] = function() { return i * 2; };
    })(i);
}
callbacks[0]() === 0;
callbacks[1]() === 2;
callbacks[2]() === 4;

With the new let in ES6, we can get rid of the function call, because let has proper block scoping.

let callbacks = []
for (let i = 0; i <= 2; i++) {
    callbacks[i] = function () { return i * 2 }
}
callbacks[0]() === 0
callbacks[1]() === 2
callbacks[2]() === 4

See how let improves the IIFE in JavaScript.

I’m not saying the let in Scheme and JavaScript are the same, but there is clearly connection between the two.

I heard that JavaScript is a bastard of Scheme and Java, now finally JavaScript added what was in Scheme decades ago. Welcome home, kid.

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