hashmap.js

// started 8:00 - 8:45/9:00 (while chatting w/ friends)

const START_SIZE = 10;
const BUCKET_LIMIT = 3;

class HashMap {
    constructor() {
        this.values = new Array(START_SIZE);
        this.count = START_SIZE;
        this.length = START_SIZE;
    }

    hash(value) {
        switch (typeof (value)) {
            case 'number':
                return value % this.length;
            case 'string':
                return value.toString()
                    .split("")
                    .reduce((sum, x) => sum + x.charCodeAt(0), 0) % this.length;
            default:
                throw new Error("Unexpected type. Valid types include: string, number.");
        }
    }

    // if this.values is filled or if there are 3 elements in a given bucket we rebalance
    // the values, rehashing and expanding this.values
    rebalance() {
        this.length = this.length ** 2;
        this.count = this.length;
        const valuesCopy = this.values;
        this.values = new Array(this.length);

        for (let bucket of valuesCopy) {
            if (bucket) {
                for (let slot of bucket) {
                    this.set(value);
                }
            }
        }
    }

    // returns an element if it exists
    // must be O(1) 
    get(value) {
        return this.values[this.hash(value)]
            .indexOf(value) !== -1;
    }

    // adds an element
    // must be O(1)
    set(value) {
        const index = this.hash(value);
        // for rebalancing, we need to know:
        //      1. if the index's values are overflowing (3)
        //      2. if this is the first value at an index
        if (this.values[index]) {
            this.values[index].push(value);
            if (this.values[index].length === BUCKET_LIMIT) {
                this.rebalance();
            }
        } else {
            this.values[index] = [value];
            this.count--;
        }
    }
}

Notes

• In interviews always try to be as explict as possible. this.length = this.length ** 2; => this.length = Math.pow(this.length, 2);
• Try to break up your logic into multiple lines for readabliity

Before

 return value.toString()
                    .split("")
                    .reduce((sum, x) => sum + x.charCodeAt(0), 0) % this.length;

After

const index = value.toString().split("").reduce((sum, x) => (sum + x.charCodeAt(0)), 0) 
return index % this.length;

• Use guarding logic to prevent nested structures.

Before

for (let bucket of valuesCopy) {
    if (bucket) {
        for (let slot of bucket) {
            this.set(value);
        }
    }
}

After

for (let bucket of valuesCopy) {
  if (!bucket) { continue }
  for (let slot of bucket) {
    this.set(value);
  }
}

• Test your code. I think this code returns a boolean but not the value. This is unexpected behavior. IndexOf() always returns a boolean. Also If this.hash(value) returns an index not in the this.values then this function will return an error.

// returns an element if it exists
    // must be O(1) 
    get(value) {
        return this.values[this.hash(value)]
            .indexOf(value) !== -1;
    }