how to create a new array from two arrays, and take from one the length and from other the values?

I have two arrays:

list_a = [20, 20, 20, 21, 21, 22, 20,21, 23, 20, 23, 24]
list_b = [120, 121, 122, 123, 124]

and I want to create the next array from those two arrays like so:

newList = [120, 120, 120, 121, 121, 122, 120, 121, 123, 120, 123, 124]

so the first number from the first array will replaced by value with 120, but not in the length of the 20, and so on, and afterwards when 20 is also located, it will replace it with the first number also

I tried a lot of stuff, nothing worked...

You could take an object for the values and keep an index of unseen values.

<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
const
list_a = [20, 20, 20, 21, 21, 22, 20, 21, 23, 20, 23, 24],
list_b = [120, 121, 122, 123, 124],
result = list_a.map(
((values, i) => v => values[v] ?? (values[v] = list_b[i++]))
({}, 0)
);

console.log(...result);
console.log(...[120, 120, 120, 121, 121, 122, 120, 121, 123, 120, 123, 124]);

<!-- end snippet -->

The specs were not clear

Try this, we create a unique list of list_a and use the placement of each to index into list_b

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const list_a = [20, 20, 20, 21, 21, 22, 20, 21, 23, 20, 23, 24]
const list_b = [120, 121, 122, 123, 124]
const idxList = [...new Set(list_a)]; // unique and placements
const newList = list_a.map(num => {
  const idx = idxList.indexOf(num)
  return list_b[idx]
});

console.log(newList)

<!-- end snippet -->

There's probably a more elegant solution with Sets but the first thing that came to my mind was to make a hash map first and then map through the list_a array.

This solution allows for any values in list_b but requires that the following condition is met:

unique_values_list_b.length >= unique_values_list_a

Hope this helps or sends you in the right direction.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

(function main () {
	const list_a = [20, 20, 20, 21, 21, 22, 20,21, 23, 20, 23, 24]
	const list_b = [120, 121, 122, 123, 124]

	let i = 0
	const hash = new Map([])

	// setting up a hash
	for(let a of list_a){
		if(hash.has(a)) continue
	
		hash.set(a, list_b[i++])		
	}


	// creating your new list

	const newList = list_a.map(element => hash.get(element))
	console.log(newList)
})()

<!-- end snippet -->

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let list_a = [20, 20, 20, 21, 21, 22, 20, 21, 23, 20, 23, 24];
let list_b = [120, 121, 122, 123, 124];

let output = list_a.map((e) => {
  return parseInt('1' + e);
});

console.log(output);

<!-- end snippet -->