What is the value of 0!? code example

Example: Factorial

// METHOD ONE
const factorialNumber = num => {
    let factorials = []
    for(let i = 1; i <= num; i++) factorials.push(i)
    return factorials.reduce((acc , curr) => acc * curr, 1)
}

// METHOD TWO
const factorialNumber = num => {
    let factorial = 1, i = 1
    while(i <= num){ factorial *= i; i++ }
    return factorial
}
// METHOD THREE
function factorialNumber(num) {
    if(num < 1) return 1
    else return factorialNumber(num - 1) * num
}