Utility function for asserting if a string is a valid float. This assumes integers are not floats.

isFloatString.ts

function isFloatString(subject: unknown): boolean {
  if (typeof subject !== 'string') {
    return false
  }
  return /^[+-]?\d+(\.\d+)$/.test(subject)
}

// tests

const tables = [
  table('3.33', true),
  table('3', false),
  table(undefined, false),
  table('3.333.33', false)
]

describe('isFloatString(subject: unknown): boolean', () => {
  tables.forEach(t => {
    it(`returns ${JSON.stringify(t.expectation)} when given ${JSON.stringify(t.input)}`, () => {
      const result = isFloatString(t.input)
      expect(result).toEqual(t.expectation)
    })
  })
})

function table(input, expectation) { return { input, expectation } }