Properly joins paths, also works for URLs

pathjoin.js

export const pathJoin = (base, ...parts) => {
    const trimSepRight = /\/+$/g;
    const trimSepLeftRight = /^\/+|\/+$/g;
    const partsTrimmed = parts.map(p => p.replace(trimSepLeftRight, ''));
    return [base.replace(trimSepRight, '')].concat(partsTrimmed).join('/');
};

for Typescript:

export const pathJoin = (base: string, ...parts: string[]) => {
  const trimSepRight = /\/+$/g;
  const trimSepLeftRight = /^\/+|\/+$/g;
  const partsTrimmed = parts.map(p => p.replace(trimSepLeftRight, ''));
  return [base.replace(trimSepRight, '')].concat(partsTrimmed).join('/');
};