Utility function that returns true if it receives a valid int -- not an int string.

isInt.ts

function isInt(subject: unknown): boolean {
  if (typeof subject !== 'number') {
    return false
  }
  const str = String(subject)
  return /^[0-9]+$/.test(str)
}

// tests

const tables = [
  table(3.33, false),
  table(3, true),
  table(33, true),
  table('33', false, 'returns false for int strings'),
  table(undefined, false),
  table('3.333.33', false)
]

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

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