DOM 04. Numpad Calculator

function solve() {
    let append = ''
    let btn = Array.from(document.querySelectorAll('button'))
    .map(x => x.addEventListener('click', (e) => {
        let press = e.target.value
        let showInput = document.getElementById('expressionOutput')
        let result = document.getElementById('resultOutput')
        if(press === 'Clear'){
            showInput.innerHTML = ''
            result.innerHTML = ''
            append = ''
        }
        else if(press === '=' && append.length > 1){
            let num = append.split(' ')
            if(num[2] === ''){
                result.innerHTML = 'NaN'
            }
            let sum = eval(`${num[0]} ${num[1]} ${num[2]}`)
             result.innerHTML = sum
        }
        else{
            if(press !== '+' && press !== '-' && press !== '/' && press !== '*'){
            append += press
            showInput.innerHTML = append
            }
            else{
                append += " " + press + ' '
                showInput.innerHTML = append
            }
        }
    }))
}