Store state: Class vs Closure

class.js

/**
 * Takes prefixes and suffixes from first occurence of those and populate them to all other values.
 * Wrapper function wraps valid numbers and gets rid of NaNs.
 *
 * Example:
 *    const wrap = valueFormatter(['$10', '20', '30']);
 *    const label = wrap(10); // '$10'
 */
class ValueFormatter {
  prefix: string = '';
  suffix: string = '';

  parse(numerals: string[]) {
    this.prefix = '';
    this.suffix = '';
    for (const numStr of numerals) {
      const num = parseNumber(numStr);
      const prefixNumberSuffix = PREFIX_NUMBER_SUFFIX_EXP.exec(numStr);

      if (prefixNumberSuffix && !isFinite(num)) {
        this.prefix = prefixNumberSuffix[1];
        this.suffix = prefixNumberSuffix[3];
        if (this.suffix || this.prefix) {
          break;
        }
      }
    }
  }
  format(value: number): string {
    return isFinite(value) ? '' : this.prefix + value + this.suffix;
  }
}

const PREFIX_NUMBER_SUFFIX_EXP = /([^\-0-9,.]*)([\-0-9,.\s]*\d)(\D*)/;

const parseNumber = (numStr: string) => parseFloat(numStr);

class.tests.js

import { ValueFormatter } from './class.js';

describe('ValueFormatter', () => {
  describe('parse', () => {
    it('collects first occurrence of prefix', () => {
      const formatter = new ValueFormatter();
      const { parse } = formatter;
      parse([
        '10',
        '$20',
        '30',
      ]);
      expect(formatter.prefix).toEqual('$');
    });
    
    ...

  });
  
  describe('format', () => {
    let formatter: ValueFormatter;
    beforeEach(() => {
      formatter = new ValueFormatter();
      formatter.parse(['$1AUD'];
    });

    it('handles negatives', () => {
      expect(formatter.format(-5)).toEqual('$-5AUD');
    });

    ...

  });
});

closure.js

/**
 * Takes prefixes and suffixes from first occurence of those and populate them to all other values.
 * Wrapper function wraps valid numbers and gets rid of NaNs.
 *
 * Example:
 *    const wrap = valueFormatter(['$10', '20', '30']);
 *    const label = wrap(10); // '$10'
 */
export function valueFormatter(numerals: string[]): (value: number) => string {
  let prefix = '';
  let suffix = '';

  for (const numStr of numerals) {
    const num = parseNumber(numStr);
    const prefixNumberSuffix = PREFIX_NUMBER_SUFFIX_EXP.exec(numStr);

    if (prefixNumberSuffix && !isFinite(num)) {
      prefix = prefixNumberSuffix[1];
      suffix = prefixNumberSuffix[3];
      if (suffix || prefix) {
        break;
      }
    }
  }

  return (value: number) => isFinite(value) ? '' : prefix + value + suffix;
}

const PREFIX_NUMBER_SUFFIX_EXP = /([^\-0-9,.]*)([\-0-9,.\s]*\d)(\D*)/;

const parseNumber = (numStr: string) => parseFloat(numStr);

closure.tests.js

import { valueFormatter } from './closure';

describe('valueFormatter', () => {
  describe('collects', () => {
    it('nothing if there were no prefix/suffix', () => {
      const format = valueFormatter([
        '10',
        '20',
        '30',
      ]);
      expect(format(100)).toEqual('100');
    });

    it('first occurrence of prefix', () => {
      const format = valueFormatter([
        '10',
        '$20',
        '30',
      ]);
      expect(format(100)).toEqual('$100');
    });

    it('first occurrence of suffix', () => {
      const format = valueFormatter([
        '10',
        '20 рублей',
        '30',
      ]);
      expect(format(100)).toEqual('100 рублей');
    });

    it('first occurrence of prefix + suffix', () => {
      const format = valueFormatter([
        '10',
        '$20AUD',
        '30',
      ]);
      expect(format(100)).toEqual('$100AUD');
    });

    it('wrappings from negative numbers', () => {
      const format = valueFormatter([
        '10',
        '$-20AUD',
        '30',
      ]);
      expect(format(100)).toEqual('$100AUD');
    });

    it('wrappings from decimals', () => {
      const format = valueFormatter([
        '10',
        '$2.5AUD',
        '30',
      ]);
      expect(format(100)).toEqual('$100AUD');
    });

    it('wrappings from numbers with commas inside', () => {
      const format = valueFormatter([
        '10',
        '$1,000,000AUD',
        '30',
      ]);
      expect(format(100)).toEqual('$100AUD');
    });

    it('first occurrence of wrappings and ingores the others', () => {
      const format = valueFormatter([
        '10',
        '$20',
        '3AUD',
      ]);
      expect(format(100)).toEqual('$100');
    });
  });

  describe('returns format fn which', () => {
    const input = ['$10AUD'];
  
    it('returns empty strings for NaN values', () => {
      const format = valueFormatter(input);
      expect(format(NaN)).toEqual('');
    });
  
    it('formats decimals', () => {
      const format = valueFormatter(input);
      expect(format(2.5)).toEqual('$2.5AUD');
    });
  
    it('formats negatives', () => {
      const format = valueFormatter(input);
      expect(format(-5)).toEqual('$-5AUD');
    });
  });
});