SOURCE

const arr = [1,2,[3,[4,5]],6]

const result = arr.flat(Infinity)
console.log(result)

const result2 = JSON.stringify(arr).replace(/\[|\]/g, '').split(',')
console.log(result2)

const result3 = JSON.parse('['+JSON.stringify(arr).replace(/\[|\]/g, '')+']')
console.log(result3)

const result4 = []
const fn = arr => {
    for(let i=0; i<arr.length; i++) {
        if(Array.isArray(arr[i])) {
            fn(arr[i])
        } else {
            result4.push(arr[i])
            console.log(result4)
        }
    }
}
console.log(fn(arr))
// console.log(result4)