关于 js forEach 的疑问?

以下代码在 chrome 输出 1,2,3

const arr = [1,2,3]
arr.forEach(i=>{
    arr.push(i)
    console.log(i)
})

这个在网上找到了,forEach 一开始就已经获取了 数组长度

The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn.

但是怎么解释以下代码 只输出了 1?

const arr = [1,2,3]
arr.forEach(i=>{
    if(i===1) arr.length = 0
    console.log(i)
})

网上都查过了, 找不到期望有大佬 能贴出源码