Generated by XState Viz: https://xstate.js.org/viz

machine.js

let game = Machine({
  id: 'game',
  initial: 'idle',
  context: {
    addends: [],
    completions: [],
    selections: []
  },
  states: {
    idle: {
      on: {
        START: 'playing'
      }
    },
    playing: {
      after: {
        1000: [
          {
            target: 'gameover',
            cond: reachedMaxAddends
          },
          {
            target: 'playing',
            actions: assign({
              addends: appendAddend
            })
          }
        ]
      },
      on: {
        SELECT: [
          {
            target: 'playing',
            cond: selectionsEqualTen,
            actions: [
              assign({
                selections: addCompletion,
                selections: []
              })
            ]
          },
          {
            target: 'gameover',
            cond: selectionsGreaterThan10
          },
          {
            cond: reachedMaxSelections,
            target: 'gameover'
          },
          {
            target: 'playing',
            cond: selectionsLessThanTen,
            actions: assign({
              selections: appendSelection,
              addends: removeAddend
            })
          }
        ]
      }
    },
    gameover: {
      on: {
        START_OVER: {
          target: 'playing',
          actions: assign({
            addends: [],
            selections: [],
            completions: []
          })
        }
      }
    }
  }
})

function reachedMaxAddends(ctx) {
  return ctx.addends.length === 8
}

function appendAddend(ctx, event) {
  return [Math.floor(Math.random() * 10), ...ctx.addends]
}

function reachedMaxSelections(ctx, event) {
  return ctx.selections === 2
}

function selectionsLessThanTen(ctx, event) {
  return ctx.selections.reduce((sum, s) => sum + s, 0) < 10
}

function appendSelection(ctx, event) {
  return [...ctx.selections, event.selection]
}

function removeAddend(ctx, event) {
  let index = ctx.addends.indexOf(event.selection)
  return ctx.addends.filter((_, i) => i !== index)
}

function selectionsGreaterThan10(ctx, event) {
  return ctx.selections.reduce((s, n) => s + n, event.selection) > 10
}

function selectionsEqualTen(ctx, event) {
  return ctx.selections.reduce((s, n) => s + n, event.selection) === 10
}

function addCompletion(ctx, event) {
  return ctx.completions.concat(ctx.selections)
}