noUnusedExpressions

import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v2.2.5` - Diagnostic Category: [`lint/suspicious/noUnusedExpressions`](/reference/diagnostics#diagnostic-category) - This rule isn't recommended, so you need to enable it. - This rule doesn't have a fix. - The default severity of this rule is [**warning**](/reference/diagnostics#warning). - Sources: - Same as [`no-unused-expressions`](https://eslint.org/docs/latest/rules/no-unused-expressions)

How to configure

json

{
	"linter": {
		"rules": {
			"suspicious": {
				"noUnusedExpressions": "error"
			}
		}
	}
}

Description

Disallow expression statements that are neither a function call nor an assignment.

When an expression is used as a statement, it should be explicitly clear what the intention behind the expression is. This is clear for function calls and assignments, because the call or the assignment itself is the primary intention behind the statement. For other expression kinds, the intention is much more ambiguous; it could be the expression contains side-effects that are not very explicit, but it could also be that it is an error where the author forgot to use the result of the expression, such as a forgotten return keyword, or it could point to a function that the author forgot to call.

Examples

Invalid

if(0) 0

<pre class="language-text"><code class="language-text">code-block.js:1:7 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ if(0) 0 │ ^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

{0}

<pre class="language-text"><code class="language-text">code-block.js:1:2 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ {0} │ ^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

f(0), {}

a && b()

a, b()

c = a, b

a() && function namedFunctionInExpressionContext () {f();}

<pre class="language-text"><code class="language-text">code-block.js:1:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ a() && function namedFunctionInExpressionContext () {f();} │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

(function anIncompleteIIFE () {});

injectGlobal`body{ color: red; }`

Set<number>

<pre class="language-text"><code class="language-text">code-block.ts:1:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ Set<number> │ ^^^^^^^^^^^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

1 as number

<pre class="language-text"><code class="language-text">code-block.ts:1:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ 1 as number │ ^^^^^^^^^^^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

window!

<pre class="language-text"><code class="language-text">code-block.ts:1:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ window! │ ^^^^^^^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

JSX expressions are considered invalid when used as a statement too:

jsx

<MyComponent />

<pre class="language-text"><code class="language-text">code-block.jsx:1:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. > 1 │ <MyComponent /> │ ^^^^^^^^^^^^^^^ 2 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

jsx

<></>

Valid

{} // In this context, this is a block statement, not an object literal

{ myLabel: foo() } // In this context, this is a block statement with a label and expression, not an object literal

function namedFunctionDeclaration () {}

(function aGenuineIIFE () {}());

f()

a = 0

new C

delete a.b

void a

Handling of Directives

Any stand-alone string at the start of a script, module, or function is considered a directive and is therefore allowed.

"use strict";
"use asm"
"use stricter";
"use babel"
"any other strings like this in the directive prologue";
"this is still the directive prologue";

function foo() {
    "bar";
}

class Foo {
    someMethod() {
        "use strict";
    }
}

The following are not considered valid directives:

doSomething();
"use strict"; // this isn't in a directive prologue, because there is a non-directive statement before it

<pre class="language-text"><code class="language-text">code-block.js:2:1 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. 1 │ doSomething(); > 2 │ "use strict"; // this isn't in a directive prologue, because there is a non-directive statement before it │ ^^^^^^^^^^^^^ 3 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

function foo() {
    "bar" + 1;
}

<pre class="language-text"><code class="language-text">code-block.js:2:5 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. 1 │ function foo() { > 2 │ "bar" + 1; │ ^^^^^^^^^ 3 │ } 4 │ ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

class Foo {
    static {
        "use strict"; // class static blocks do not have directive prologues
    }
}

<pre class="language-text"><code class="language-text">code-block.js:3:9 <a href="https://biomejs.dev/linter/rules/no-unused-expressions">lint/suspicious/noUnusedExpressions</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Expected an assignment or function call but found an expression instead. 1 │ class Foo { 2 │ static { > 3 │ "use strict"; // class static blocks do not have directive prologues │ ^^^^^^^^^^^^^ 4 │ } 5 │ } ℹ This expression may be unintentionally unused or it might be a function that you forgot to call. </code></pre>

</TabItem> </Tabs>