How to fix the forbidden non-null assertion in TypeScript and React?

Matthew C.

—June 15, 2024

The ProblemThe Problem

When using typescript-eslint in a JavaScript application, you may encounter the following error:

@typescript-eslint/no-non-null-assertion
warning: Forbidden non-null assertion

The following example code would cause this error:

const loggedInUsername = "bob";

const users = [
  { name: "Steven", age: 12 },
  { name: "Lisa", age: 32 }
];

const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser!.age);

You can see a live demo of this error in this typescript-eslint playground.

For this error to occur, the eslintrc file has the “no-non-null-assertion” rule set to "error":

"rules": {
  "@typescript-eslint/no-non-null-assertion": "error"
}

The Non-null Assertion Operator (Postfix !) assertion removes the null and undefined types from a value. To use it, add the ! symbol after an expression like in the console log example above. When your TypeScript configuration is set to do "strictNullChecks", use the non-null assertion operator to bypass the null and undefined type checks. It should only be used if you know a specific value won’t be null or undefined at runtime.

The SolutionThe Solution

Your error message may include a solution to the error:

Consider using the optional chain operator `?.` instead. This operator includes runtime checks, so it is safer than the compile-only non-null assertion operator.

To fix the example code above, use the optional chain operator when accessing the age property of the loggedInUser:

console.log(loggedInUser?.age);

If the age property does not exist on the loggedInUser object, the expression will result in undefined instead of throwing an error.