The function λx.x x is the self application function. It applies its argument to its argument.

// λx.x x
const sa = x => x(x)

Like the identity function, it doesn't seem helpful at first glance but it has some interesting applications. Consider the twice function λf.λx.f (f x), which applies function f to the value x and then applies function f again to the result of that.

// λf.λx.f (f x)

const twice = f => x => f(f(x))
const addOne = a => a + 1

twice(addOne)(1) // 3

By supplying the twice function with the addOne function, and then supplying the resulting function with 1, we get 3. That's because twice(addOne)(1) is the same as addOne(addOne(1)) or 1 + 1 + 1.

Going back to the self application function, since it applies its argument to its argument, given the function twice, it will apply twice to twice, applying the addOne function four times.

// λx.x x
const sa = x => x(x)

// λf.λx.f (f x)
const twice = f => x => f(f(x))

// (λx.x x) (λf.λx.f (f x))
const fourTimes = sa(twice)
const addOne = a => a + 1
fourTimes(addOne)(1) // 5