Is there a way to create a type that ensures each character is either '1' or '0' in JavaScript?
type Binary = '1'|'0'
The current type works for single characters only. Is it possible to create a type for repeating patterns of '1' and '0' characters?
Furthermore, I am interested in enforcing a specific length. The following implementation is all I could come up with:
type BinaryInput = `${Binary}${Binary}${Binary}${Binary}${Binary}`;
If you are looking for type safety at the interfaces of your API, consider utilizing a type that extends string and cannot be obtained without assertion (referred to as a "snowflake type"). You can then implement a type guard to validate the strings, or directly assert the type if you are certain and require the performance benefits of skipping the runtime check.
type Binary<N extends number = number> = string & {
readonly BinaryStringLength: unique symbol;
length: N;
};
class AssertionError extends Error {
name = 'AssertionError';
}
function assertIsBinaryString <L extends number>(
str: string,
length?: L,
): asserts str is Binary<L> {
if (typeof length === 'number' && str.length !== length) {
throw new AssertionError('Binary string has invalid length');
}
for (const unit of str) {
if (unit !== '0' && unit !== '1') {
throw new AssertionError('Invalid character in binary string');
}
}
}
// Implementation:
function asDecimal (bStr: Binary): number {
return parseInt(bStr, 2);
}
function byteAsChar (byte: Binary<8>): string {
return new TextDecoder().decode(new Uint8Array([asDecimal(byte)]));
}
let str = '00100100';
// console.log(asDecimal(str)); // string not assignable to Binary
/* ^^^
Error 2345 */
assertIsBinaryString(str);
console.log(asDecimal(str)); // now works correctly => 36
// console.log(byteAsChar(str)); // Binary<number> not assignable to Binary<8> because number not assignable to 8
/* ^^^
Error 2345 */
assertIsBinaryString(str, 8);
console.log(byteAsChar(str)); // now works correctly => "$"
Choosing branded types may be the optimal choice for your situation.
If you are storing binary data as a number, like this:
const myBinary = 0b1000;
typeof myBinary // 'number'
You have the option to create a branded primitive type for number:
type Binary = number & { readonly __brand: unique symbol };
To specify a specific length for the binary data:
type Binary<Length extends number> = number & { readonly __brand: unique symbol, length: Length }
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