sexp-template/src/env.js

module.exports = class Env {
  constructor(parent = null) {
    if (parent) {
      this.data = parent.data
    } else {
      this.data = {
        '+': (a, b) => a + b,
        '-': (a, b) => a - b,
        '*': (a, b) => a * b,
        '/': (a, b) => a / b,
      }
    }
  }

  get(symbol) {
    if (this.data[symbol]) {
      return this.data[symbol]
    }

    throw `Symbol ${symbol} is not bound`
  }

  set(symbol, value) {
    this.data[symbol] = value
  }
}