--- description: 'Disallow non-null assertion in locations that may be confusing.' --- import Tabs from '@theme/Tabs'; import TabItem from '@theme/TabItem'; > 🛑 This file is source code, not the primary documentation location! 🛑 > > See **https://typescript-eslint.io/rules/no-confusing-non-null-assertion** for documentation. Using a non-null assertion (`!`) next to an assignment or equality check (`=` or `==` or `===`) creates code that is confusing as it looks similar to an inequality check (`!=` `!==`). ```typescript a! == b; // a non-null assertion(`!`) and an equals test(`==`) a !== b; // not equals test(`!==`) a! === b; // a non-null assertion(`!`) and a triple equals test(`===`) ``` Using a non-null assertion (`!`) next to an in test (`in`) or an instanceof test (`instanceof`) creates code that is confusing since it may look like the operator is negated, but it is actually not. {/* prettier-ignore */} ```typescript a! in b; // a non-null assertion(`!`) and an in test(`in`) a !in b; // also a non-null assertion(`!`) and an in test(`in`) !(a in b); // a negated in test a! instanceof b; // a non-null assertion(`!`) and an instanceof test(`instanceof`) a !instanceof b; // also a non-null assertion(`!`) and an instanceof test(`instanceof`) !(a instanceof b); // a negated instanceof test ```` This rule flags confusing `!` assertions and suggests either removing them or wrapping the asserted expression in `()` parenthesis. ## Examples ```ts interface Foo { bar?: string; num?: number; } const foo: Foo = getFoo(); const isEqualsBar = foo.bar! == 'hello'; const isEqualsNum = 1 + foo.num! == 2; ``` {/* prettier-ignore */} ```ts interface Foo { bar?: string; num?: number; } const foo: Foo = getFoo(); const isEqualsBar = foo.bar == 'hello'; const isEqualsNum = (1 + foo.num!) == 2; ``` ## When Not To Use It If you don't care about this confusion, then you will not need this rule. ## Further Reading - [`Issue: Easy misunderstanding: "! ==="`](https://github.com/microsoft/TypeScript/issues/37837) in [TypeScript repo](https://github.com/microsoft/TypeScript)