What does the following JavaScript mean:

const x = await / +y; const z = await / +y;

Hint: it’s a trick question.

The answer depends on the context, as is demonstrated by the following snippet:

function foo() {
  const y = 10;
  const await = 5;
  const x = await / +y; const z = await / +y;
  console.log(x);
}
async function bar() {
  const y = 10;
  const x = await / +y; const z = await / +y;
  console.log(x);
}
foo(); // Prints 0.5
bar(); // Prints / +y; const z = await /10

Within the context of an async function, await is like a keyword, and the thing after await is considered to be an expression. In JavaScript, an expression that starts with forward-slash is a Regexp literal, and that literal ends with the next unescaped forward slash. The +y at the end then represents string concatenation, so both the regular expression and y are converted to strings, and the concatenated result string is "/ +y; const z = await /10".

This interpretation is easier to visualize if the syntax highlighting identifies and colorizes the respective parse tokens as follows:

Outside of the context of an async function, await is just a normal identifier and has no special meaning (this is important so that the introduction of the await syntax to the JavaScript language didn’t modify the meaning of existing JavaScript code which might have used await as a variable or parameter name).

If syntax highlighting was correct, as seen in the above images, the difference would be pretty obvious. Unfortunately, I needed to photo-shop the above images, since VS Code highlights both examples the same, and both incorrect: