Aggregation applies not only to numbers but also to strings. These are tasks in which a string is built dynamically, that is, it is not known in advance how big it will be or what it will contain.

Imagine a function that can "multiply" a string, that is, it repeats it a specified number of times:

repeat('hexlet', 3); // hexlethexlethexlet

The way this function works is quite simple: in a loop, the string is "grown" the specified number of times:

const repeat = (text, times) => {
  // The neutral element for strings is the empty string
  let result = '';
  let i = 1;

  while (i <= times) {
    // Each time we add the string to the result
    result = `${result}${text}`;
    i = i + 1;
  }

  return result;
};

Let's break down the execution of this code step by step:

// For the call repeat('hexlet', 3);
let result = '';
result = `${result}hexlet`; // hexlet
result = `${result}hexlet`; // hexlethexlet
result = `${result}hexlet`; // hexlethexlethexlet

Visually, the process of growing the string looks like this:

repeat('hexlet', 3):

i=1: result = ''             + 'hexlet' = 'hexlet'
i=2: result = 'hexlet'       + 'hexlet' = 'hexlethexlet'
i=3: result = 'hexlethexlet' + 'hexlet' = 'hexlethexlethexlet'
                                            └── result

Neutral element

For growing to work, a starting value is needed. For strings, this value is the empty string ''.

It is called the neutral element because it changes nothing during concatenation:

console.log('' + 'abc'); // => abc
console.log('abc' + ''); // => abc

That is why the empty string is always used as the initial value when aggregating strings.

Implement a function sanitizePhoneNumber() that takes a phone number from a form and returns a string without spaces, parentheses, and hyphens.

Users enter numbers in different ways, but before saving them they are normalized to a single format. Go through the original string character by character and assemble a new number using only the useful characters.

sanitizePhoneNumber('+7 (999) 123-45-67'); // => '+79991234567'
sanitizePhoneNumber('8 800 555 35 35');    // => '88005553535'
sanitizePhoneNumber('(123) 456-7890');     // => '1234567890'

Use the empty string as the initial value.