SOURCE

function jisuan() {
    let result = document.querySelector('#result')
    const input = document.querySelector('#num')
    let value = input.value
    if(value.length<4){
        alert('请输入四位数')
        return
    }
    // let sum = Array.from(value).reduce((cur, pre)=>{
    //     return cur*pre
    // }, 1)
    let sum = 1
    for(let i=0,len = value.length;i<len;i++){
        console.log(value[i])
        sum *= value[i]
    }
     result.innerHTML = sum
}


let button = document.querySelector('#button')
button.addEventListener('click', jisuan)

四位数：<input maxlength="4" id="num" type="number" />
<button id="button">计算</button>
<div id="result"></div>