Arrays have mutating side effects

[ghost]

const array = [1,2,3,4]
const immuArray = immu(array)
console.log(immuArray.push(5)) // [1,2,3,4,5]
array.pop()
console.log(immuArray.push(5)) // [1,2,3,5]

This is due to the array operations operating on the mutable source array. See arr in immutableArray
https://github.com/scottcorgan/immu/blob/master/src/index.js#L54