Just because the standard path module didn't feel easy enough.

setDir(s) replaces all dirs.

const p1 = new FilePath('a/b/c.md')
const p2 = p1.setDir('x/y')
// p1 == 'a/b/c.md'
// p2 == 'x/y/c.md'

setFile changes the name of the (without the extension).

const p1 = new FilePath('a/b/c.md')
const p2 = p1.setFile('x')
// p1 == 'a/b/c.md'
// p2 == 'a/b/x.md'

setExt changes just the extension of the path

const p1 = new FilePath('a/b/c.md')
const p2 = p1.setExt('html')
// p1 == 'a/b/c.md'
// p2 == 'a/b/c.html'

push(s) appends directories at the end.

const p1 = new FilePath('a/b/c.md')
const p2 = p1.push('d/e')
// p1 == 'a/b/c.md'
// p2 == 'a/b/d/e/c.md'

pop(n) strips n directory levels from the end.

const p1 = new FilePath('a/b/c.md')
const p2 = p1.pop(1)
// p1 == 'a/b/c.md'
// p2 == 'a/c.md'

shift(n) strips n directory levels from the beginning.

const p1 = new FilePath('a/b/c.md')
const p2 = p1.shift(1)
// p1 == 'a/b/c.md'
// p2 == 'b/c.md'

unshift(s) prefixes directories at the beginning.

const p1 = new FilePath('a/b/c.md')
const p2 = p1.unshift('x/y')
// p1 == 'a/b/c.md'
// p2 == 'x/y/a/b/c.md'