SOURCE

let arr = [1, [2, 3, [4, 5]]]
function flat1(arr) {
    let arrs = []
    while(arr.some(item => Array.isArray(item))) {
        arr = [].concat(...arr)
    }
    console.log(arr)
}

function flat2(arr) {
    const arrs = []
    while(arr.length) {
        const a = arr.shift();
        if (Array.isArray(a)) {
            arr.unshift(...a)
        } else {
            arrs.push(a)
        }
    }
    console.log(arrs)
}

function flat3(arr) {
    let arrs = []; 
    arr.forEach(item => {
        if (Array.isArray(item)) {
            arrs.push(...flat3(item))
        } else {
            arrs.push(item)
        }
    })
    return arrs
}

function flat4(arr) {
    return arr.reduce((acc, cur) => {
        if (Array.isArray(cur)) {
            return [...acc, ...flat4(cur)]
        } else {
            return [...acc, cur]
        }
    }, [])
}

// flat1(arr)
// flat2(arr)
console.log(flat3(arr))
// console.log(flat4(arr))