Typescript function that generates all permutations of strings

Write a typescript function that generates all permutations of a string array and outputs them concatenated with a separator. For example:

let items = ["this", "is", "cool"]
let separator = "/"

let result = createPermutations(items, separator)

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Result would be:

result = ["this/is/cool", "this/cool/is", "is/this/cool", "is/cool/this", "cool/this/is", "cool/is/this"]

The order of items in the array does not matter.