Building off of#1 andthis comment, the great advantage oflet/const is its block-scoping, i.e.:

leta=1;if(true){letb=2;}console.log(b);// undefined

This is a dramatic improvement overvar, and a big reason whylet andconst have become popular features. This block scoping is probably something that CoffeeScript should have, separate from the feature ofconst that means “throw an error on reassignment.”

We could have both, via:= and:== operators (or whatever two operators people think are best):

• a := 1 would mean, “declarea at the top of this block usinglet, and on this line assigna with the value of1.”
• a :== 1 would mean, “declare and assigna on this line usingconst. If it gets reassigned later, throw an error.”

We don’t necessarily need the second operator, if we don’t care to give the “throw an error on reassignment” feature.

let has the same issue asvar, in that its declaration is hoisted to its entire scope (whatMDN refers to as thetemporal dead zone), solet declarations should be grouped together at the top of their block scope similar to howvar declarations are currently grouped at the top of their function scope.const must be declared and assigned on the same line, so it can’t get moved up.

So this CoffeeScript:

a=1b :=2c :==3if (yes)  b :=4  c :==5

would compile into this #"auto" data-snippet-clipboard-copy-content="var a;let b;a = 1;b = 2;const c = 3;if (true) { let b; b = 4; const c = 5;}">

vara;letb;a=1;b=2;constc=3;if(true){letb;b=4;constc=5;}