parser.js

const src = `
var a = 1;
var b:string = '123'
interface User {
 name: string
}
`;

const getDataBetweenChars = (start, end, data) => {
  const lines = cleanArray(data.split('\n'))
  let flag = false;
  let newData = '';
  cleanArray(lines.map(line => {
    if (contains(line, start)) {
      flag = true
    } else if (contains(line, end)) {
      flag = false
    }
    
    newData = flag 
    ? newData + line
    : newData;
  }));
  
 return newData
        .replace(start, '')
        .replace(stop, '');
}

const contains = (s, c) => {
  return s.indexOf(c) > -1
}

const cleanArray = (a) => {
  return a.filter(x => x)
}

console.log(getDataBetweenChars('interface', '}', src));