addIndex.js

import _concat from './internal/_concat';
import _curry1 from './internal/_curry1';
import curryN from './curryN';


/**
 * Creates a new list iteration function from an existing one by adding two new
 * parameters to its callback function: the current index, and the entire list.
 *
 * This would turn, for instance, [`R.map`](#map) function into one that
 * more closely resembles `Array.prototype.map`. Note that this will only work
 * for functions in which the iteration callback function is the first
 * parameter, and where the list is the last parameter. (This latter might be
 * unimportant if the list parameter is not used.)
 *
 * @func
 * @memberOf R
 * @since v0.15.0
 * @category Function
 * @category List
 * @sig ((a ... -> b) ... -> [a] -> *) -> ((a ..., Int, [a] -> b) ... -> [a] -> *)
 * @param {Function} fn A list iteration function that does not pass index or list to its callback
 * @return {Function} An altered list iteration function that passes (item, index, list) to its callback
 * @example
 *
 *      const mapIndexed = R.addIndex(R.map);
 *      mapIndexed((val, idx) => idx + '-' + val, ['f', 'o', 'o', 'b', 'a', 'r']);
 *      //=> ['0-f', '1-o', '2-o', '3-b', '4-a', '5-r']
 */
var addIndex = _curry1(function addIndex(fn) {
  return curryN(fn.length, function() {
    var idx = 0;
    var origFn = arguments[0];
    var list = arguments[arguments.length - 1];
    var args = Array.prototype.slice.call(arguments, 0);
    args[0] = function() {
      var result = origFn.apply(this, _concat(arguments, [idx, list]));
      idx += 1;
      return result;
    };
    return fn.apply(this, args);
  });
});
export default addIndex;