# functional/no-expression-statements 📝 Disallow expression statements. 💼🚫 This rule is enabled in the following configs:  `noStatements`, ✅ `recommended`, 🔒 `strict`. This rule is _disabled_ in the ☑️ `lite` config. 💭 This rule requires [type information](https://typescript-eslint.io/linting/typed-linting). This rule checks that the value of an expression is assigned to a variable and thus helps promote side-effect free (pure) functions. ## Rule Details When you call a function and don’t use it’s return value, chances are high that it is being called for its side effect. e.g. ### ❌ Incorrect ```js /* eslint functional/no-expression-statements: "error" */ console.log("Hello world!"); ``` ```js /* eslint functional/no-expression-statements: "error" */ array.push(3); ``` ```js /* eslint functional/no-expression-statements: "error" */ foo(bar); ``` ### ✅ Correct ```js /* eslint functional/no-expression-statements: "error" */ const baz = foo(bar); ``` ```js /* eslint functional/no-expression-statements: ["error", { "ignoreVoid": true }] */ console.log("hello world"); ``` ## Options This rule accepts an options object of the following type: ```ts type Options = { ignoreCodePattern?: string[] | string; ignoreVoid?: boolean; ignoreSelfReturning?: boolean; }; ``` ### Default Options ```ts const defaults = { ignoreVoid: false, ignoreSelfReturning: false, }; ``` ### `ignoreVoid` When enabled, expression of type `void` and `Promise` are not flagged as violations. This options requires TypeScript in order to work. ### `ignoreSelfReturning` Like `ignoreVoid` but instead does not flag function calls that always only return `this`. Limitation: The function declaration must explicitly use `return this`; equivalents (such as assign this to a variable first, that is then returned) won't be considered valid. ### `ignoreCodePattern` This option takes a RegExp string or an array of RegExp strings. It allows for the ability to ignore violations based on the code itself.