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noStringCaseMismatch

src/content/docs/linter/rules/no-string-case-mismatch.mdx

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

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v1.0.0` - Diagnostic Category: [`lint/correctness/noStringCaseMismatch`](/reference/diagnostics#diagnostic-category) - This rule is **recommended**, meaning it is enabled by default. - This rule has an [**unsafe**](/linter/#unsafe-fixes) fix. - The default severity of this rule is [**error**](/reference/diagnostics#error). - Sources: - Same as [`match_str_case_mismatch`](https://rust-lang.github.io/rust-clippy/master/#match_str_case_mismatch)

How to configure

json
{
	"linter": {
		"rules": {
			"correctness": {
				"noStringCaseMismatch": "error"
			}
		}
	}
}

Description

Disallow comparison of expressions modifying the string case with non-compliant value.

Examples

Invalid

js
if (s.toUpperCase() === "Abc") {}
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-string-case-mismatch">lint/correctness/noStringCaseMismatch</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">This expression always returns false.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (s.toUpperCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This call convert the string to upper case</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (s.toUpperCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">... but this value is not in upper case</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>if (s.toUpperCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Use </span><span style="color: lightgreen;"><strong>upper case</strong></span><span style="color: lightgreen;"> string value.</span> <strong>1</strong> <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">i</span><span style="color: Tomato;">f</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">(</span><span style="color: Tomato;">s</span><span style="color: Tomato;">.</span><span style="color: Tomato;">t</span><span style="color: Tomato;">o</span><span style="color: Tomato;">U</span><span style="color: Tomato;">p</span><span style="color: Tomato;">p</span><span style="color: Tomato;">e</span><span style="color: Tomato;">r</span><span style="color: Tomato;">C</span><span style="color: Tomato;">a</span><span style="color: Tomato;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">(</span><span style="color: Tomato;">)</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;">=</span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">&quot;</span><span style="color: Tomato;"><strong>A</strong></span><span style="color: Tomato;"><strong>b</strong></span><span style="color: Tomato;"><strong>c</strong></span><span style="color: Tomato;">&quot;</span><span style="color: Tomato;">)</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">&#123;</span><span style="color: Tomato;">&#125;</span> <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">U</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">p</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">C</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">&quot;</span><span style="color: MediumSeaGreen;"><strong>A</strong></span><span style="color: MediumSeaGreen;"><strong>B</strong></span><span style="color: MediumSeaGreen;"><strong>C</strong></span><span style="color: MediumSeaGreen;">&quot;</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">&#123;</span><span style="color: MediumSeaGreen;">&#125;</span> <strong>2</strong> <strong>2</strong><strong> │ </strong> </code></pre>
js
while (s.toLowerCase() === "Abc") {}
<pre class="language-text"><code class="language-text">code-block.js:1:8 <a href="https://biomejs.dev/linter/rules/no-string-case-mismatch">lint/correctness/noStringCaseMismatch</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ <strong><span style="color: Tomato;">✖</span></strong> <span style="color: Tomato;">This expression always returns false.</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>while (s.toLowerCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This call convert the string to lower case</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>while (s.toLowerCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">... but this value is not in lower case</span> <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>while (s.toLowerCase() === &quot;Abc&quot;) &#123;&#125; <strong> │ </strong> <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong> <strong>2 │ </strong> <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Use </span><span style="color: lightgreen;"><strong>lower case</strong></span><span style="color: lightgreen;"> string value.</span> <strong>1</strong> <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">w</span><span style="color: Tomato;">h</span><span style="color: Tomato;">i</span><span style="color: Tomato;">l</span><span style="color: Tomato;">e</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">(</span><span style="color: Tomato;">s</span><span style="color: Tomato;">.</span><span style="color: Tomato;">t</span><span style="color: Tomato;">o</span><span style="color: Tomato;">L</span><span style="color: Tomato;">o</span><span style="color: Tomato;">w</span><span style="color: Tomato;">e</span><span style="color: Tomato;">r</span><span style="color: Tomato;">C</span><span style="color: Tomato;">a</span><span style="color: Tomato;">s</span><span style="color: Tomato;">e</span><span style="color: Tomato;">(</span><span style="color: Tomato;">)</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;">=</span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">&quot;</span><span style="color: Tomato;"><strong>A</strong></span><span style="color: Tomato;"><strong>b</strong></span><span style="color: Tomato;"><strong>c</strong></span><span style="color: Tomato;">&quot;</span><span style="color: Tomato;">)</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">&#123;</span><span style="color: Tomato;">&#125;</span> <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">w</span><span style="color: MediumSeaGreen;">h</span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">l</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">L</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">w</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">C</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">&quot;</span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>b</strong></span><span style="color: MediumSeaGreen;"><strong>c</strong></span><span style="color: MediumSeaGreen;">&quot;</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">&#123;</span><span style="color: MediumSeaGreen;">&#125;</span> <strong>2</strong> <strong>2</strong><strong> │ </strong> </code></pre>

Valid

js
if (s.toUpperCase() === "ABC") {}
while (s.toLowerCase() === "abc") {}
for (;s.toLocaleLowerCase() === "ABC";) {}
while (s.toLocaleUpperCase() === "abc") {}
for (let s = "abc"; s === "abc"; s = s.toUpperCase()) {}
</TabItem> </Tabs>