E0374: unexpected whitespace between '!' and '=='

x! == y is a typo for a strict inequality, as in x !== y:

let x = 17;
let y = 38;
if (x! == y) {
    alert('Not equal!');
}

To fix the warning, replace ! == with !==:

let x = 17;
let y = 38;
if (x !== y) {
  alert("Not equal!");
}

See also: E0373

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