To find the people in the ancestry data set who were young in 1924, the following function might be helpful. It filters out the elements in an array that don’t pass a test.

filter-array.js

// ancestry = http://eloquentjavascript.net/code/ancestry.js
function filter(array, test) {
  var passed = [];
  for (var i = 0; i < array.length; i++) {
    if (test(array[i]))
      passed.push(array[i]);
  }
  return passed;
}

console.log(filter(ancestry, function(person) {
  return person.born > 1900 && person.born < 1925;
}));

/*
Transforming with map
Say we have an array of objects representing people, produced by filtering the ancestry array somehow.
But we want an array of names, which is easier to read.
The map method transforms an array by applying a function to all of its elements and building a new 
array from the returned values. The new array will have the same length as the input array, but its 
content will have been “mapped” to a new form by the function.
*/
function map(array, transform){
  var mapped = [];
  for (var i = 0; i < array.length; i++){
    mapped.push(transform(array[i]));
  }
  return mapped;
}

var overNinty = ancestry.filter(function(person){
	return person.died - person.born > 90;
});

console.log(map(overNinty, function(person){
	return person.name;
}));