Semantic predicates and label visibility in PEG.js

gistfile1.txt

/*
 * Task: Parse even numbers in PEG.js (http://pegjs.majda.cz).
 *
 * Solution: Let's parse all numbers and reject odd ones using a semantic
 * predicate -- an arbitrary piece of code that returns true (meaning "continue
 * parsing") or false (meaning "halt parsing").
 *
 * This solution wouldn't work before commit a2af1fe612 because predicates
 * didn't have access to labeled expressions in the grammar as variables
 * (without ugly workarounds). But they have the access now and the solution
 * works.
 */

// Whitespace-separated numbers
numbers = number (" \t\r\n"+ number)*

// One (even) number
number = digits:[0-9]+ &{ return parseInt(digits.join(""), 10) % 2 === 0; }