parser function for sbn compiler

parser.js

function parser (tokens) {
  var AST = {
    type: 'Drawing',
    body: []
  }
  // extract a token at a time as current_token. Loop until we are out of tokens.
  while (tokens.length > 0){
    var current_token = tokens.shift()

    // Since number token does not do anything by it self, we only analyze syntax when we find a word.
    if (current_token.type === 'word') {
      switch (current_token.value) {
        case 'Paper' :
          var expression = {
            type: 'CallExpression',
            name: 'Paper',
            arguments: []
          }
          // if current token is CallExpression of type Paper, next token should be color argument
          var argument = tokens.shift()
          if(argument.type === 'number') {
            expression.arguments.push({  // add argument information to expression object
              type: 'NumberLiteral',
              value: argument.value
            })
            AST.body.push(expression)    // push the expression object to body of our AST
          } else {
            throw 'Paper command must be followed by a number.'
          }
          break
        case 'Pen' :
          ...
        case 'Line':
          ...
      }
    }
  }
  return AST
}