Is there are a "const" version in the specification language?

1. Let _iterator_ be the **this** value.

Is it possible to indicate the spec reader, that the iterator variable will not change in the following algorithm?

1. Let const _iterator_ to be the **this** value.

Immutable constants can reduce the reading effort. (Also need support from the linter to ban the re-assignment on the const variable, and the spec render to mark it visually different from the let which is mutable).

(And I think we're re-inventing JS in the spec language)

Nope. The specification language is intentionally pretty simple.