js check if character is letter

1
2
3
4
5
const isLetter = (character) => {
    // it will work only for: most Latin, Greek, Armenian and Cyrillic scripts.
    // it will not work for: Chinese, Japanese, Arabic, Hebrew and most other scripts.
    return character && character.length === 1 && character.toLowerCase() !== character.toUpperCase();
};

1
2
3
4
5
const LETTER_EXPRESSION = /^\p{L}$/u; // Supported by ES6+, Some bugs in FF < 78

const isLetter = (character) => {
    return character && LETTER_EXPRESSION.test(character);
};

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
let isLetter;

try {
    const LETTER_EXPRESSION = /^\p{L}$/u; // Supported by ES6+, Some bugs in FF < 78

    isLetter = (character) => {
        return character && LETTER_EXPRESSION.test(character);
    };
} catch(e) {
  	console.log('Unicode flag in regular expressions are not supported. Used legacy isLetter() function.');

    isLetter = (character) => {
      	// it doesn't work for some alphabets but still enough in most cases
        return character && character.length === 1 && character.toLowerCase() !== character.toUpperCase();
    };
}


// Usage example:

//Note: Unicode expression is supported by modern Chrome, Safar, Edge, FF

//                            // Unicode expression / Case conversion (legacy)

// letters

console.log(isLetter('é'));   // true               / true
console.log(isLetter('è'));   // true               / true
console.log(isLetter('ê'));   // true               / true
console.log(isLetter('ü'));   // true               / true
console.log(isLetter('ö'));   // true               / true
console.log(isLetter('à'));   // true               / true
console.log(isLetter('a'));   // true               / true
console.log(isLetter('z'));   // true               / true
console.log(isLetter('A'));   // true               / true
console.log(isLetter('Z'));   // true               / true
console.log(isLetter('日'));  // true               / false
console.log(isLetter('я'));   // true               / true
console.log(isLetter('דֹ'));   // false              / false

// digits

console.log(isLetter('0'));  // false               / false
console.log(isLetter('5'));  // false               / false
console.log(isLetter('9'));  // false               / false

// other

console.log(isLetter('10')); // false               / false
console.log(isLetter('ab')); // false               / false
console.log(isLetter('😎')); // false               / false

console.log(isLetter('-'));  // false               / false
console.log(isLetter('.'));  // false               / false
console.log(isLetter('+'));  // false               / false
console.log(isLetter('*'));  // false               / false