Using await in a function that isn't async is illegal in JavaScript, so Prettier currently fails to format with a parser error. However, Prettier should be able to detect an await inside a function that doesn't have async and add it when formatting for convenience and to prevent confusion.

Prettier 2.1.1
Playground link

Input:

async function depenency() {}

function dependent() {
  await dependency();
}

Output:

SyntaxError: Unexpected token, expected ";" (4:9)
  2 | 
  3 | function dependent() {
> 4 |   await dependency()
    |         ^
  5 | }

Expected behavior:

async function depenency() {}

async function dependent() {
  await dependency();
}