Value for Key Path -- Deep Object Property Inspection

example.js

var x = { 
          a: { 
               b: [ 
                    { c: 2, d: 3 },
                    { c: 3, d: 4 } 
                  ]
             }, 
          f: null
        };

Object.prototype.valueForKeyPath = valueForKeyPath;

console.log ( JSON.stringify(x.valueForKeyPath( "a.b[1].c", 99 )) );

valueForKeyPath.js

/**
 * Returns a value for the specified keypath. If any intervening
 * values evaluate to undefined or null, the entire result is
 * undefined or null, respectively.
 *
 * If you need a default value to be returned in such an instance,
 * specify it after the keypath.
 *
 * Note: if `o` is not an object, it is assumed that the function
 * has been bound to `this`. As such, all arguments are shifted by
 * one position to the right.
 *
 * Key paths are of the form:
 *
 *    object.field.field.field[index]
 *
 * @param {object} o        the object to search
 * @param {string} k        the keypath
 * @param {*} d             (optional) the default value to return
 *                          should the keypath evaluate to null or
 *                          undefined.
 * @return {*}              the value at the keypath
 * 
 * License MIT: Copyright 2014 Kerri Shotts
 */
function valueForKeyPath ( o, k, d ) {
  if (o === undefined || o === null) { 
    return (d !== undefined) ? d : o; 
  }
  if ( !(o instanceof Object) ) {
    d = k;
    k = o;
    o = this;
  }
  var v = o;
  // There's a million ways that this regex can go wrong
  // with respect to JavaScript identifiers. Splits will
  // technically work with just about every non-A-Za-z\$-
  // value, so your keypath could be "field/field/field"
  // and it would work like "field.field.field".
  v = k.match(/([\w\$\\\-]+)/g)
      .reduce( function ( v, keyPart ) {
       if (v === undefined || v === null) {
         return v;
       }
       try {
         return v[ keyPart ];
       } catch ( err ) {
         return undefined;
       }
     }, v );
  return ((v === undefined || v === null) &&
          (d !== undefined) ) ? d : v;
}