How to do case insensitive string comparison in JavaScript ?

To perform a case-insensitive string comparison in JavaScript, you can use several methods depending on your use case (equality check, sorting, or handling locale-specific characters). Below are detailed approaches with examples:

1. Convert Strings to Same Case & Compare

The simplest method: convert both strings to uppercase or lowercase and compare.

Example 1: Check Equality

const str1 = "HELLO";
const str2 = "hello";

// Using toLowerCase()
console.log(str1.toLowerCase() === str2.toLowerCase()); // true

// Using toUpperCase()
console.log(str1.toUpperCase() === str2.toUpperCase()); // true

Example 2: In Conditional Statements

function areEqualCaseInsensitive(a, b) {
  return a.toLowerCase() === b.toLowerCase();
}

console.log(areEqualCaseInsensitive("Apple", "APPLE")); // true

Limitations:

Fails for certain Unicode characters (e.g., Turkish İ).
Not suitable for locale-aware comparisons.

2. Use `localeCompare()` for Locale-Sensitive Comparisons

The localeCompare() method with the sensitivity: 'accent' or sensitivity: 'base' option handles case insensitivity and locale-specific characters.

Example 3: Basic Case-Insensitive Check

const strA = "café";
const strB = "CAFÉ";

// Returns 0 if equal (case-insensitive)
console.log(strA.localeCompare(strB, undefined, { sensitivity: 'base' })); // 0

Example 4: Check Equality

function areEqualLocale(a, b) {
  return a.localeCompare(b, undefined, { sensitivity: 'base' }) === 0;
}

console.log(areEqualLocale("groß", "GROSS")); // false (due to ß vs. SS)
console.log(areEqualLocale("İ", "i")); // false (Turkish İ vs. i)

Parameters:

sensitivity: 'base': Ignores case and accents (e.g., a vs. À).
sensitivity: 'accent': Ignores case but considers accents (e.g., a ≠ À).

3. Use Regular Expressions (`RegExp`)

Use a regular expression with the i (case-insensitive) flag to test equality.

Example 5: Exact Match

const str1 = "Hello";
const str2 = "HELLO";

// Check if strings are identical (case-insensitive)
const isEqual = new RegExp(`^${str1}$`, 'i').test(str2);
console.log(isEqual); // true

Example 6: Partial Match

const text = "The Quick Brown Fox";
console.log(/quick/i.test(text)); // true

4. Handling Turkish Locale Issues

The Turkish İ (U+0130) and ı (U+0131) characters break case conversion. Use localeCompare with an explicit locale.

Example 7: Turkish Case Comparison

const trStr1 = "İSTANBUL";
const trStr2 = "istanbul";

// Incorrect with toLowerCase():
console.log(trStr1.toLowerCase() === trStr2.toLowerCase()); // false

// Correct with localeCompare():
console.log(
  trStr1.localeCompare(trStr2, 'tr-TR', { sensitivity: 'base' }) === 0
); // true

5. Case-Insensitive Sorting

Use localeCompare with sensitivity: 'base' to sort arrays case-insensitively.

Example 8: Sort an Array

const words = ["Banana", "apple", "Cherry"];
words.sort((a, b) => a.localeCompare(b, undefined, { sensitivity: 'base' }));
console.log(words); // ["apple", "Banana", "Cherry"]

Summary Table

Method	Use Case	Locale Support	Key Notes
`toLowerCase()`/`toUpperCase()`	Simple equality checks	❌ No	Fails for Unicode edge cases
`localeCompare()`	Locale-aware equality/sorting	✅ Yes	Handles Turkish `İ` and others
`RegExp` with `i`	Pattern matching or exact equality	❌ No	Limited to regex pattern checks

Key Takeaways

For Basic Checks: Use toLowerCase() or toUpperCase() with ===.
For Locale-Aware Comparisons: Use localeCompare() with sensitivity: 'base'.
For Turkish/Unicode Edge Cases: Specify the locale explicitly (e.g., 'tr-TR').
For Pattern Matching: Use RegExp with the i flag.

By choosing the right method, you can handle case insensitivity accurately in JavaScript!