feat(lowerFirst): add lowerFirst function (#403)

* feat(lowerFirst): Add lowerFirst function

* Add lowerFirst benchmark

* Add lowerFirst english docs

---------

Co-authored-by: Sojin Park <raon0211@toss.im>

benchmarks/performance/lowerFirst.bench.ts

@@ -0,0 +1,28 @@
+import { bench, describe } from 'vitest';
+import { lowerFirst as lowerFirstToolkit } from 'es-toolkit';
+import { lowerFirst as lowerFirstLodash } from 'lodash';
+
+describe('lowerFirst', () => {
+  describe('short string', () => {
+    bench('es-toolkit/lowerFirst', () => {
+      const str = 'camelCase';
+      lowerFirstToolkit(str);
+    });
+
+    bench('lodash/lowerFirst', () => {
+      const str = 'camelCase';
+      lowerFirstLodash(str);
+    });
+  });
+
+  describe('long string', () => {
+    const LONG_STR = 'camelCaseLongString'.repeat(1000);
+    bench('es-toolkit/lowerFirst', () => {
+      lowerFirstToolkit(LONG_STR);
+    });
+
+    bench('lodash/lowerFirst', () => {
+      lowerFirstLodash(LONG_STR);
+    });
+  });
+});

docs/reference/string/lowerFirst.md

@@ -0,0 +1,26 @@
+# lowerFirst
+
+Converts the first character of string to lower case.
+
+## Signature
+
+```typescript
+function lowerFirst(str: string): string;
+```
+
+### Parameters
+
+- `str` (`string`): The string that is to be changed.
+
+### Returns
+
+(`string`) The string converted.
+
+## Examples
+
+```typescript
+import { lowerFirst } from 'es-toolkit/string';
+
+upperCase('Fred') // returns 'fred'
+upperCase('FRED') // returns 'fRED'
+```

src/string/index.ts

@@ -5,3 +5,4 @@ export { lowerCase } from './lowerCase.ts';
 export { startCase } from './startCase.ts';
 export { capitalize } from './capitalize.ts';
 export { pascalCase } from './pascalCase.ts';
+export { lowerFirst } from './lowerFirst.ts';

src/string/lowerFirst.spec.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { lowerFirst } from './lowerFirst';
+
+describe('lowerFirst', () => {
+  it('should change first character of upper case to lower case', () => {
+    expect(lowerFirst('Fred')).toEqual('fred');
+  });
+
+  it('should keep first character of lower case unaltered', () => {
+    expect(lowerFirst('fred')).toEqual('fred');
+  });
+
+  it('should keep all other characters unaltered', () => {
+    expect(lowerFirst('FRED')).toEqual('fRED');
+  });
+
+  it('should work with an empty string', () => {
+    expect(lowerFirst('')).toEqual('');
+  });
+
+  it('should keep white space and not alter string', () => {
+    expect(lowerFirst('  fred')).toEqual('  fred');
+  });
+});

src/string/lowerFirst.ts

@@ -0,0 +1,14 @@
+/**
+ * Converts the first character of string to lower case.
+ *
+ * @param {string} str - The string that is to be changed
+ * @returns {string} - The converted string.
+ *
+ * @example
+ * const convertedStr1 = lowerCase('fred') // returns 'fred'
+ * const convertedStr2 = lowerCase('Fred') // returns 'Fred'
+ * const convertedStr3 = lowerCase('FRED') // returns 'FRED'
+ */
+export const lowerFirst = (str: string): string => {
+  return str.substring(0, 1).toLowerCase() + str.substring(1);
+};