Js unit test

head.js

function head(x) {
  if (typeof x === 'string' && x.length > 0) {
    return x.charAt(0)
  } else if (Array.isArray(x) && x.length > 0) {
    return x[0]
  }
  return null
}

module.exports = {
  head
}

head.test.js

const head = require('./head').head
const assert = require('chai').assert

describe('head', function() {
  it('returns first item if x is array', function() {
    assert.equal(1, head([1, 2, 3]))
  })

  it('returns null if x is array but length is 0', function() {
    assert.isNull(head([]))
  })

  it('returns first character if x is string', function() {
    assert.equal('a', head('abc'))
  })

  it('returns null if x is string but length is 0', function() {
    assert.isNull(head(''))
  })

  it('returns null if neither x is null', function() {
    assert.isNull(head(null))
  })

  it('returns null if neither x is string nor array', function() {
    assert.isNull(head(123))
  })
})