ES6 In Depth: let and const

The feature I’d like to talk about today is at once humble and startlingly ambitious. When Brendan Eich designed the first version of JavaScript back in 1995, he got plenty of things wrong, including things that have been part of the language ever since, like the Date object and objects automatically converting to NaN when you accidentally multiply them. However, the things he got right are stunningly important things, in hindsight: objects; prototypes; first-class functions with lexical scoping; mutability by default. The language has good bones. It was better than anyone realized at first. Still, Brendan made one particular design decision that bears on today’s article—a decision that I think can be fairly characterized as a mistake. It’s a little thing. A subtle thing. You might use the language for years and not even notice it. But it matters, because this mistake is in the side of the language that we now think of as “the good parts”. It has to do with variables.