A CodePen by Darren Torpey. Real-world-y underscore.js example

script.js

function report(name, data) {
    console.log('===============');
    console.log(name, data);
    console.log(JSON.stringify(data));
}

var jsonData = [
  {
    id:   1,
    name: 'Darren',
    role: 'Developer'
  },
  {
    id:   2,
    name: 'Audrey',
    role: 'Developer'
  },
  {
    id:   3,
    name: 'Eric',
    role: 'QA'
  },
  {
    id:   5,
    name: 'Mike',
    role: 'Developer'
  },
  {
    id:   6,
    name: 'Bev',
    role: 'PM'
  },
  {
    id:   6,
    name: 'Frank',
    role: 'PM'
  },
  {
    id:   12,
    name: 'Bryan',
    role: 'QA'
  }
];
report('Start:', jsonData);

var techLookup = _.reduce(jsonData, function(newObj, pair) {
  newObj[pair.name] = pair;
  return newObj;
}, {});
report('techLookup:', techLookup);
/*
{
  "Darren":{
    "id":1,
    "name":"Darren",
    "role":"Developer"
  },
  "Audrey":{
    "id":2,
    "name":"Audrey",
    "role":"Developer"
  },
  "Eric":{
    "id":3,
    "name":"Eric",
    "role":"QA"
  },
  "Mike":{
    "id":5,
    "name":"Mike",
    "role":"Developer"
  },
  "Bev":{
    "id":6,
    "name":"Bev",
    "role":"PM"
  },
  "Frank":{
    "id":6,
    "name":"Frank",
    "role":"PM"
  },
  "Bryan":{
    "id":12,
    "name":"Bryan",
    "role":"QA"
  }
}
*/

var roleLookup = _.reduce(jsonData, function(roleMap, person) {
  roleMap[person.role] = roleMap[person.role] || [];
  roleMap[person.role].push(person);
  return roleMap;
}, {});
report('roleLookup', roleLookup);
/*
{
  "Developer":[
    {
      "id":1,
      "name":"Darren",
      "role":"Developer"
    },
    {
      "id":2,
      "name":"Audrey",
      "role":"Developer"
    },
    {
      "id":5,
      "name":"Mike",
      "role":"Developer"
    }
  ],
  "QA":[
    {
      "id":3,
      "name":"Eric",
      "role":"QA"
    },
    {
      "id":12,
      "name":"Bryan",
      "role":"QA"
    }
  ],
  "PM":[
    {
      "id":6,
      "name":"Bev",
      "role":"PM"
    },
    {
      "id":6,
      "name":"Frank",
      "role":"PM"
    }
  ]
}
*/

var uberRoleLookup = _.reduce(jsonData, function(roleMap, person) {
  roleMap[person.role] = roleMap[person.role] || {};
  roleMap[person.role][person.name] = person;
  return roleMap;
}, {});
report('uberRoleLookup', uberRoleLookup);
/*
{
  "Developer":{
    "Darren":{
      "id":1,
      "name":"Darren",
      "role":"Developer"
    },
    "Audrey":{
      "id":2,
      "name":"Audrey",
      "role":"Developer"
    },
    "Mike":{
      "id":5,
      "name":"Mike",
      "role":"Developer"
    }
  },
  "QA":{
    "Eric":{
      "id":3,
      "name":"Eric",
      "role":"QA"
    },
    "Bryan":{
      "id":12,
      "name":"Bryan",
      "role":"QA"
    }
  },
  "PM":{
    "Bev":{
      "id":6,
      "name":"Bev",
      "role":"PM"
    },
    "Frank":{
      "id":6,
      "name":"Frank",
      "role":"PM"
    }
  }
}
*/

var controversialRoleLookup = _.reduce(jsonData, function(roleMap, person) {
  roleMap = _.defaults(roleMap, _.object([[person.role, {}]]));
  roleMap[person.role][person.name] = person;
  return roleMap;
}, {});
report('controversialRoleLookup', controversialRoleLookup);