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TypeScript - check if character is letter
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In this short article, we would like to show how to check if the character is a letter using TypeScript.
Hint: in the below example you will find a solution that has support for i18n for many alphabets.
It is like
\wregular expression group with i18n characters.
Quick solutions:
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const isLetter = (character: string): boolean => {2
// it will work only for: most Latin, Greek, Armenian and Cyrillic scripts.3
// it will not work for: Chinese, Japanese, Arabic, Hebrew and most other scripts.4
return character && character.length === 1 && character.toLowerCase() !== character.toUpperCase();5
};or:
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const LETTER_EXPRESSION: RegExp = /^\p{L}$/u; // Supported by ES6+, Some bugs in FF < 782
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const isLetter = (character: string): boolean => {4
return character && LETTER_EXPRESSION.test(character);5
};
Important things:
- by default, only modern TypeScript (ES6+) introduces some solutions to check if some character is a letter,
- if ES6+ is not supported, it is possible to use case conversion to make some letter detection - it doesn't work in all cases but can be enough,
- the third option is to use an external library: XRegExp or unicode-properties (with
getCategory()) - but it increases bundle size, what not accepted on some websites.
In this section, you will find a function that gives good enough support for many letters detection.
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// ONLINE-RUNNER:browser;2
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let isLetter;4
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try {6
const LETTER_EXPRESSION: RegExp = /^\p{L}$/u; // Supported by ES6+, Some bugs in FF < 787
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isLetter = (character: string): boolean => {9
return character && LETTER_EXPRESSION.test(character);10
};11
} catch (e) {12
console.log('Unicode flag in regular expressions are not supported. Used legacy isLetter() function.');13
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isLetter = (character: string): boolean => {15
// it doesn't work for some alphabets but still enough in most cases16
return character && character.length === 1 && character.toLowerCase() !== character.toUpperCase();17
};18
}19
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// Usage example:22
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//Note: Unicode expression is supported by modern Chrome, Safar, Edge, FF24
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// // Unicode expression / Case conversion (legacy)26
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// letters28
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console.log(isLetter('é')); // true / true30
console.log(isLetter('è')); // true / true31
console.log(isLetter('ê')); // true / true32
console.log(isLetter('ü')); // true / true33
console.log(isLetter('ö')); // true / true34
console.log(isLetter('à')); // true / true35
console.log(isLetter('a')); // true / true36
console.log(isLetter('z')); // true / true37
console.log(isLetter('A')); // true / true38
console.log(isLetter('Z')); // true / true39
console.log(isLetter('日')); // true / false40
console.log(isLetter('я')); // true / true41
console.log(isLetter('דֹ')); // false / false42
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// digits44
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console.log(isLetter('0')); // false / false46
console.log(isLetter('5')); // false / false47
console.log(isLetter('9')); // false / false48
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// other50
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console.log(isLetter('10')); // false / false52
console.log(isLetter('ab')); // false / false53
console.log(isLetter('😎')); // false / false54
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console.log(isLetter('-')); // false / false56
console.log(isLetter('.')); // false / false57
console.log(isLetter('+')); // false / false58
console.log(isLetter('*')); // false / false59
