# E0261: TypeScript non-null assertions are not allowed in JavaScript

In TypeScript, `!` after a expression is a non-null assertion. It is a syntax error to write a non-null assertion in JavaScript:

    /** @type {string} favoritePowerRanger */
    let favoritePowerRanger;
    if (player.hasFavoriteColor) {
      favoritePowerRanger = player.favoriteColor!;
    } else {
      favoritePowerRanger = "(unknown)";
    }

To fix this error, remove the `!`:

    /** @type {string} favoritePowerRanger */
    let favoritePowerRanger;
    if (player.hasFavoriteColor) {
      favoritePowerRanger = player.favoriteColor;
    } else {
      favoritePowerRanger = "(unknown)";
    }

Introduced in quick-lint-js version 2.7.0.

Documentation for other errors