Lisp parser

// Lisp parser. Written in JavaScript/ES6. 

const expressionStart = (token) => token[0]  === '(';

const expressionEnd = (token) => token[token.length - 1]  === ')';

function evaluate(token) {
  let numbered = Number(token);

  if (token === 'true') {
    return true;

  } else if (token === 'false') {
    return false;

  // if coercing the token into a number doesn't return NaN
  } else if (numbered == token) {
    return numbered;

  } else {
    return token;
  }
}

function findExpressionSize(tokens) {
  let parenCount = 1;
  let ix = 1;

  for (; parenCount; ix++) {
    const token = tokens[ix];

    if (expressionStart(token)) {
      parenCount += token.match(/\(/g).length;

    } else if (expressionEnd(token)) {
      parenCount -= token.match(/\)/g).length;
    }
  }

  return ix;
}

function parse(string) {
  const output = [];

  const expression = string.slice(1, -1);
  const tokens = expression.split(' ');

  for (let i = 0; i < tokens.length; i++) {
    const token = tokens[i];

    if (expressionStart(token)) {
      // How many tokens ahead does the expression end in?
      const expressionSize = findExpressionSize(tokens.slice(i));
      const expressionSubset = tokens.slice(i, i + expressionSize).join(' ');
      output.push( parse(expressionSubset) );

      i += expressionSize - 1;

    } else {
      output.push( evaluate(token) );
    }
  }
  
  return output;
}

let lisp = '(+ 1 2 (- 4 (- 5 4) (- 1 0)) (- 6 1 (+ 1 1) 1) 5)';
parse(lisp);
// => 
// [ '+',
//    1,
//    2,
//    [ '-', 4, [ '-', 5, 4 ], [ '-', 1, 0 ] ],
//    [ '-', 6, 1, [ '+', 1, 1 ], 1 ],
//    5 ]