file.ts

function mul(vec0: number[], vec1: number[]): number[] {
    if (vec0.length < vec1.length) {
        const temp = vec0
        vec0 = vec1
        vec1 = temp
    }

    const results: number[][] = []

    // numbers0.length >= numbers1
    for (let i = 0; i < vec1.length; i++) {
        results.push(mul_single(vec0, vec1[i], i))
    }

    return results.reduce((vec0, vec1) => _merge(vec0, vec1), [0])
}

// return new multiplied vector without modifing original
function mul_single(vec: number[], multiplier: number, exponent: number): number[] {
    vec = [...vec]
    for (let i = 0; i < vec.length; i++) {
        vec[i] = vec[i] * multiplier
    }

    return new Array(exponent).map(() => 0).concat(vec)
}

// in-place merge vec1 into vec0, return vec0
function _merge(vec0: number[], vec1: number[]): number[] {
    if (vec0.length < vec1.length) {
        const temp = vec0
        vec0 = vec1
        vec1 = temp
    }

    // vec0.length >= vec1.length
    for (let i = 0; i < vec1.length; i++) {
        vec0[i] = vec0[i] + vec1[i]
    }

    _normalize(vec0)

    return vec0
}

// in-place normalize vector
function _normalize(vec: number[]) {
    for (let i = 0; i < vec.length; i++) {
        if (vec[i] < 10) {
            continue
        }

        if (i + 1 == vec.length) {
            vec.push(0)
        }

        const value = vec[i]
        vec[i] = vec[i] % 10
        vec[i + 1] = Math.floor(value / 10)
    }
}

function _int_to_vec(value: number): number[] {
    const vec = []
    while (value > 0) {
        vec.push(Math.floor(value / 10))
        value = Math.floor(value / 10)
    }

    return vec
}

function parse_input(input: string): [number[], number[]] {
    const splitted = input.split(' ')

    const vec0 = [...splitted[0]].map((v) => Number.parseInt(v))
    const vec1 = [...splitted[1]].map((v) => Number.parseInt(v))

    return [vec0, vec1]
}

const input = '893724358493284 238947328947329'
const [vec0, vec1] = parse_input(input)

const result = mul(vec0, vec1)

console.log(result.reverse().map(v => v.toString()).join(''))