When to use HashMap instead of Loop

Many programmers use a loop or filter where HashMap data structure could be considered.

Finding user by id using Loops

let userIdToBeSearched = 103;
const users = [
    {id: 101, name: "Josh"},
    {id: 102, name: "Mosh"},
    {id: 103, name: "Eli"},
    {id: 104, name: "Jad"},
    {id: 105, name: "Susan"}
];

let user = null;
for (let i = 0; i < users.length; i++) {
    if (users[i].id === userIdToBeSearched) {
        user = users[i];
        break;
    }
}

if (user) {
    console.log("User Found: ", user);
} else {
    console.log("user does not exit with id: ", userIdToBeSearched);
}

The above solution has a time complexity of O(n), where n represents the number of users. If there are 1 thousand users, in the worst case, we will search every user to find a match.

Considering user id will be unique for each user, this is a good indication to use a HashMap instead of a loop since all keys in the Map are Unique.

Finding user by id using Map

let userIdToBeSearched = 103;
const users = new Map();
users.set(101, {id: 101, name: "Josh"});
users.set(102, {id: 102, name: "Mosh"});
users.set(103, {id: 103, name: "Eli"});
users.set(104, {id: 104, name: "Jad"});
users.set(105, {id: 105, name: "Susan"});

if(users.has(userIdToBeSearched)){
  console.log("User Found: ", users.get(userIdToBeSearched));
}
else {
  console.log("user does not exit with id: ", userIdToBeSearched);
}

When using a Map, it takes constant time O(1) to find the user! All great, but note constructing the HashMap from the array still requires O(n) time.

In conclusion, use Map when frequently searching based on the unique field such as id. Please note Map cannot be used in case of searching based on the non-unique field such as name

Thank you for reading! Check the part 2 where we discuss A Practical Use Case of HashMaps. When to use HashMap instead of Loop Part 2