Extending MarkLogic Sequences with Array methods

sequence-prototype.js

'use strict';
/**
 * @param {number} begin Zero-based index to start
 * @param {number} [end] Zero-based index to end. Does not support negative indexes, like `Array.prototype.slice`
 * @return {Sequence}
 */
Sequence.prototype.slice = function(begin, end) {
  if('number' !== typeof begin) { throw new TypeError('begin must be a number'); }
  if(end) {
    return fn.subsequence(this, begin + 1, end - begin + 1);
  }
  return fn.subsequence(this, begin + 1);
}

/**
 * Execute a function on each item.
 *
 * @param {function} fct
 * @param {object} [that] 
 * @return {Iterator}
 */
Sequence.prototype.map = function*(fct, that) {
  if('function' !== typeof fct) { throw new TypeError('fct must be a function'); }
  for(let item of this) {
     yield fct.call(that || null, item);
  }
}

/**
 * Lazily evaluate XPath over each item.
 *
 * @param {string} path 
 * @param {object} [bindings]
 * @param {object} [that]
 * @return {Iterator}
 */
Sequence.prototype.xpath = function*(path, bindings, that) {
  if(null === path || 'undefined' === typeof path) { throw new TypeError('path must be a string of XPath'); }
  for(let item of this) {
    if(item instanceof Node || item instanceof Document || 'function' === typeof item.xpath) {
      yield* item.xpath(path, bindings);
    }
  }
}

/**
 *
 * @callback reducer
 * @param {object}   value     The previous value
 * @param {object}   item      The current item
 * @param {number}   index     The index of the current item
 * @param {Sequence} sequence  The parent `Sequence`
 * @return {object}  The next value
 */

/**
 * Accumulate a value over each item.
 *
 * @param {reducer} fct
 * @param {object} init Initial value
 */
Sequence.prototype.reduce = function(fct, init) {
  let value = init, index = 0;
  for(let item of this) {
    value = fct.call(null, value, item, index++, this);
  }
  return value;
}

test.sjs

const seq = Sequence.from(
  [
    xdmp.unquote('<foo><bar/></foo>'), 
    xdmp.unquote('<foo><bar/></foo>'), 
    xdmp.unquote('<foo><baz/></foo>')
  ]
);

Sequence.from(
  seq.xpath('//bar')
);