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## Intro 🌐

Last time, we learned how to get data from our hash table .

Today, we’ll learn how to get the keys of our Hash Table.

## Requirements 💭

We need the following parts to get the keys from our Hash Table:

• a method to get the keys (`keys`)

## Starter Code ▶️

We start with the code with the `set` method, so that we can use the `set` method to add some data.

``````class Hashtable {
constructor() {
this.data = [];
this.size = 0;
}

hash(key) {
const chars = key.split("");
const charCodes = chars.map((char) => char.charCodeAt());
const charCodeSum = charCodes.reduce((acc, cur) => acc + cur);
return charCodeSum;
}

set(key, value) {
const hash = this.hash(key);

if (!this.data[hash]) {
this.data[hash] = [];
}

this.data[hash].push([key, value]);

this.size++;
}
}``````

If you are not familiar with the hash function, re-read this post.

## Thoughts 💭

First, we should think about the constraints and possibilities:

• first, we declare an empty array for the keys
• then we iterate over the data array
• if there is data (= array of key-value pairs) at this specific index, iterate over this data (= the single key-value pairs)
• add the data (= key) to the keys array
• return the keys array

## Example

We want to get all the keys.

``````// current hash table data:
[
[["age", 33]],
[
["name", "miku86"],
["mean", false],
],
];

// desired data:
["age", "name", "mean"];``````

## Steps

``````// current hash table data:
[
[["age", 33]],
[
["name", "miku86"],
["mean", false],
],
];

// then we iterate over the data array
[["age", 33]];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["age", 33];

// add the data (= key) to the keys array
["age"];

// then we iterate over the data array
[
["name", "miku86"],
["mean", false],
];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["name", "miku86"];

// add the data (= key) to the keys array
["age", "name"];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["mean", false];

// add the data (= key) to the keys array
["age", "name", "mean"];

// desired data:
["age", "name", "mean"];``````

## Implementation ⛑

``````// a Hash Table class
class Hashtable {
constructor() {
this.data = [];
this.size = 0;
}

hash(key) {
const chars = key.split("");
const charCodes = chars.map((char) => char.charCodeAt());
const charCodeSum = charCodes.reduce((acc, cur) => acc + cur);
return charCodeSum;
}

set(key, value) {
const hash = this.hash(key);

if (!this.data[hash]) {
this.data[hash] = [];
}

this.data[hash].push([key, value]);

this.size++;
}

keys() {
// declare an empty array for the keys
const keys = [];

// iterate over the data array (I call a single array a "bucket")
for (let bucket of this.data) {
// if there is data (= array of key-value pairs) at this specific index
if (bucket) {
// iterate over this data (= the single key-value pairs)
for (let item of bucket) {
// add the data (= key) to the keys array
keys.push(item[0]);
}
}
}

// return the keys array
return keys;
}
}``````

Note: I’m using a `for ... of`-loop. If you don’t know how this works, you can read about it on MDN. You can use whatever you want to use, a default `for`-loop, a `for ... in`-loop, a functional approach etc.

## Result

``````// create a new hash table
const newHashtable = new Hashtable();

// add three new key-value pairs
newHashtable.set("name", "miku86");
newHashtable.set("mean", false);
newHashtable.set("age", 33);

// show the hash table data
console.log(newHashtable.data);
// [ <301 empty items>, [ [ 'age', 33 ] ], <115 empty items>, [ [ 'name', 'miku86' ], [ 'mean', false ] ] ]

// show the keys
console.log(newHashtable.keys());
// [ 'age', 'name', 'mean' ] ✅``````

## Next Part ➡️

We managed to write a function to get all keys, great work!

Next time, we’ll learn how to get all values from our Hash Table.

• How would you implement the `keys`-function?