Convert string to match E.164 phone number pattern (e.g. +1234567890)

e.164.phoneNumber.js

/**
 * Convert string to match E.164 phone number pattern (e.g. +1234567890),
 * otherwise return empty string.
 */
function enforcePhoneNumberPattern(string) {
  let newString = string.match(/[0-9]{0,14}/g);

  if (newString === null) {
    return '';
  }

  // Join parts returned from RegEx match
  newString = newString.join('');

  // Start number with "+"
  newString = '+' + newString;

  // Limit length to 15 characters
  newString = newString.substring(0, 15);

  return newString;
}

First let me say thanks this almost worked for me however it would add an + sign even there was no 1 in the phone number. Here is a simple update to your function to check for a 1 for the first number and if not there add it too.

if (newString[0].includes('1')) { newString = '+' + newString; } else { newString = '+1' + newString; }

add it before line 18

First let me say thanks this almost worked for me however it would add an + sign even there was no 1 in the phone number. Here is a simple update to your function to check for a 1 for the first number and if not there add it too.

if (newString[0].includes('1')) { newString = '+' + newString; } else { newString = '+1' + newString; }

add it before line 18

A number does not always need to start with "1" to add the +-sign, this just means the country code where "1" means US. So your given solution would only work for the US and all other countries would also suddenly become US.
For example a number 32 (Belgium) --> 32 378542364 would become +132378542364 while it should be +32378542364.