expressions.js

const builtins = [
  ['+', (head=0, ...tail) => tail.reduce((acc, num) => acc += num, head)],
  ['-', (head=0, ...tail) => tail.reduce((acc, num) => acc -= num, head)],
  ['*', (head=0, ...tail) => tail.reduce((acc, num) => acc = acc * num, head)],
  ['/', (head=0, ...tail) => tail.reduce((acc, num) => acc = acc / num, head)],
  ['repeat', (head='', tail) => head.repeat(tail)],
  ['concat', (head, ...tail) => [head, ...tail]],
  ['join', (head, ...tail) => [...tail].join(head)]
]

builtins.forEach(([name, func]) => {
  window[name] = func
})

const consume = arg =>
  Array.isArray(arg)
  ? process(arg)
  : arg

const process = ([func, ...args]=['+', 0, 0]) => {
  const builtin = typeof func === 'function' ? func : window[func]
  return builtin(...args.map(arg => consume(arg)))
}

// Addition
console.log(
  process(
    ['+', 1, ['+', 2, 3]]
  )
)


// Subtraction
console.log(
  process(
    ['-', 10, 5]
  )
)

// Multiplication
console.log(
  process(
    ['*', 12, 12]
  )
)

// Division
console.log(
  process(
    ['/', 100, 50]
  )
)

// repeat
console.log(
  process(
    [repeat, 'ha ', 3]
  )
)

// concat
console.log(
  process(
    [concat, 'hello', 'world']
  )
)

// join
console.log(
  process(
    [join, ' ', 'hello', 'world']
  )
)