Javascript let

Mozilla offers a let in javascript 1.7. The let keyword isn’t support by other browsers yet so using it isn’t generally recomended but it nice to catch up on the latest javascript constructs.

A Parallel

First, a parallel. I’ve coded in Java the last couple of months because of school. During said time, I discovered scoped blocks in Java. A scoped block might be like this:

public void myMethod() {
  int i = 100, j = 10;
  {
    int k = i * j;
    i = 10;
    j = 10;
  }
  // cannot reference k from here
}

If you’re a java programmer, you might never have seen this happen before. Basically, putting floating brackets, as long as they match up, around arbitrary code, it will be in a different scope. k won’t be accessible from the outside of those brackets. I call these structures scoped blocks.

Now back to javascript

let in javascript acts similarly to the scoped blocks mentioned above.

Using let as a block is similar to the scoped block in java. The let-block won’t allow g to be accessed from the outside, x and m are newly bound values, they will only be around in the block scope.

 var i = 100, j = 10;
 let (x = 10+i, m = j + 5) {
   var g = x + m;
 }

let might make it easy to bind this to another variable in a certain scope, which could prove useful.

For more information about let, checkout the MDC page.