Block scoping

Champions: Dave Herman, Mark Miller, Allen Wirfs-Brock
Details: full proposal

Block scoping introduces new declaration forms for defining variables scoped to a single block. This includes:

  • let: syntactically similar to var, but defines a variable in the current block
  • function: allow function declarations in nested blocks
  • const: like let, but for read-only constant declarations

A better var

Using let in place of var makes it easier to define block-local variables without worrying about them clashing with variables elsewhere in the same function body. 

function log(msg) { ... }

function f(x) {
    if (...) {
        let { log, sin, cos } = Math;
        ... log(x) ...
    }
    log("done computing f()");
}

Closures in loops

Using let makes it easier to create closures within loops: 

for (i = 0; i < n; i++) {
    let x = a[i];
    element.onclick = function() {
        ... x ...
    };
}

Closures in for loops

FIXME: example