diff --git a/lib/main.js b/lib/main.js
index 511b1c3..40bf18b 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -26,7 +26,12 @@ function interpolate (value, processEnv, parsed) {
       }
 
       if (parsed[key]) {
-        return parsed[key]
+        // avoid recursion from EXPAND_SELF=$EXPAND_SELF
+        if (parsed[key] === value) {
+          return parsed[key]
+        } else {
+          return interpolate(parsed[key], processEnv, parsed)
+        }
       }
 
       if (defaultValue) {
diff --git a/tests/main.js b/tests/main.js
index 8db911c..a2b8aa9 100644
--- a/tests/main.js
+++ b/tests/main.js
@@ -9,530 +9,543 @@ t.beforeEach((ct) => {
   process.env = {}
 })
 
-// t.test('returns object', ct => {
-//   const dotenv = { parsed: {} }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   t.ok(parsed instanceof Object, 'should be an object')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables', ct => {
-//   const dotenv = {
-//     parsed: {
-//       BASIC: 'basic',
-//       BASIC_EXPAND: '${BASIC}',
-//       BASIC_EXPAND_SIMPLE: '$BASIC'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.BASIC_EXPAND, 'basic')
-//   ct.equal(parsed.BASIC_EXPAND_SIMPLE, 'basic')
-//
-//   ct.end()
-// })
-//
-// t.test('uses environment variables existing already on the machine for expansion', ct => {
-//   process.env.MACHINE = 'machine'
-//   const dotenv = {
-//     parsed: {
-//       MACHINE_EXPAND: '${MACHINE}',
-//       MACHINE_EXPAND_SIMPLE: '$MACHINE'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MACHINE_EXPAND, 'machine')
-//   ct.equal(parsed.MACHINE_EXPAND_SIMPLE, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('does not expand environment variables existing already on the machine that look like they could expand', ct => {
-//   process.env.PASSWORD = 'pas$word'
-//   const dotenv = {
-//     parsed: {
-//       PASSWORD: 'dude',
-//       PASSWORD_EXPAND: '${PASSWORD}',
-//       PASSWORD_EXPAND_SIMPLE: '$PASSWORD'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.PASSWORD_EXPAND, 'pas$word')
-//   ct.equal(parsed.PASSWORD_EXPAND_SIMPLE, 'pas$word')
-//   ct.equal(parsed.PASSWORD, 'pas$word')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string', ct => {
-//   const dotenv = {
-//     parsed: {
-//       UNDEFINED_EXPAND: '$UNDEFINED'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND, '')
-//
-//   ct.end()
-// })
-//
-// t.test('prioritizes machine key expansion over .env', ct => {
-//   process.env.MACHINE = 'machine'
-//   const dotenv = {
-//     parsed: {
-//       MACHINE: 'machine_env',
-//       MACHINE_EXPAND: '$MACHINE'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MACHINE_EXPAND, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('does not expand escaped variables', ct => {
-//   const dotenv = {
-//     parsed: {
-//       ESCAPED_EXPAND: '\\$ESCAPED'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.ESCAPED_EXPAND, '$ESCAPED')
-//
-//   ct.end()
-// })
-//
-// t.test('does not expand inline escaped dollar sign', ct => {
-//   const dotenv = {
-//     parsed: {
-//       INLINE_ESCAPED_EXPAND: 'pa\\$\\$word'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.INLINE_ESCAPED_EXPAND, 'pa$$word')
-//
-//   ct.end()
-// })
-//
-// t.test('does not overwrite preset variables', ct => {
-//   process.env.SOME_ENV = 'production'
-//   const dotenv = {
-//     parsed: {
-//       SOME_ENV: 'development'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.SOME_ENV, 'production')
-//
-//   ct.end()
-// })
-//
-// t.test('does not expand inline escaped dollar sign', ct => {
-//   const dotenv = {
-//     parsed: {
-//       INLINE_ESCAPED_EXPAND_BCRYPT: '\\$2b\\$10\\$OMZ69gxxsmRgwAt945WHSujpr/u8ZMx.xwtxWOCMkeMW7p3XqKYca'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.INLINE_ESCAPED_EXPAND_BCRYPT, '$2b$10$OMZ69gxxsmRgwAt945WHSujpr/u8ZMx.xwtxWOCMkeMW7p3XqKYca')
-//
-//   ct.end()
-// })
-//
-// t.test('handle mixed values', ct => {
-//   const dotenv = {
-//     parsed: {
-//       PARAM1: '42',
-//       MIXED_VALUES: '\\$this$PARAM1\\$is${PARAM1}'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MIXED_VALUES, '$this42$is42')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.BASIC_EXPAND, 'basic')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables (process.env)', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.BASIC_EXPAND, 'basic')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.MACHINE_EXPAND, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine (process.env)', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.MACHINE_EXPAND, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND, '')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string (process.env)', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND, '')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine even with a default', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.EXPAND_DEFAULT, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine even with a default when nested', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.EXPAND_DEFAULT_NESTED, 'machine')
-//   ct.equal(process.env.EXPAND_DEFAULT_NESTED2, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables undefined with one already on the machine even with a default when nested', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.UNDEFINED_EXPAND_NESTED, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string but replaces with default', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT, 'default')
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT2, 'default')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environent variables and concats with default nested', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE, 'machine_envdefault')
-//   ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE2, 'machine_envdefault')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environent variables and concats with default nested', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE, 'machine_envdefault')
-//   ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE2, 'machine_envdefault')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string but replaces with default nested', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED, 'default')
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED2, 'default')
-//
-//   ct.end()
-// })
-//
-// t.test('expands missing environment variables to an empty string but replaces with default nested twice', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED_TWICE, 'default')
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED_TWICE2, 'default')
-//
-//   ct.end()
-// })
-//
-// t.test('prioritizes machine key expansion over .env', ct => {
-//   process.env.MACHINE = 'machine'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MACHINE_EXPAND, 'machine')
-//
-//   ct.end()
-// })
-//
-// t.test('multiple expand', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MONGOLAB_URI, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
-//
-//   ct.end()
-// })
-//
-// t.test('should expand recursively', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MONGOLAB_URI_RECURSIVELY, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
-//
-//   ct.end()
-// })
-//
-// t.test('multiple expand', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.NO_CURLY_BRACES_URI, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
-//
-//   ct.end()
-// })
-//
-// t.test('should expand recursively', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.NO_CURLY_BRACES_URI_RECURSIVELY, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
-//
-//   ct.end()
-// })
-//
-// t.test('can write to an object rather than process.env if user provides it', ct => {
-//   const myObject = {}
-//   const dotenv = {
-//     processEnv: myObject,
-//     parsed: {
-//       SHOULD_NOT_EXIST: 'testing'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//   const evaluation = typeof process.env.SHOULD_NOT_EXIST
-//
-//   ct.equal(parsed.SHOULD_NOT_EXIST, 'testing')
-//   ct.equal(myObject.SHOULD_NOT_EXIST, 'testing')
-//   ct.equal(evaluation, 'undefined')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine even with a default with special characters', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS, 'machine_env')
-//   ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS2, 'machine_env')
-//
-//   ct.end()
-// })
-//
-// t.test('expands environment variables existing already on the machine even with a default with special characters (process.env)', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS, 'machine_env')
-//   ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS2, 'machine_env')
-//
-//   ct.end()
-// })
-//
-// t.test('should expand with default value correctly', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS, '/default/path:with/colon')
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS2, '/default/path:with/colon')
-//   ct.equal(parsed.NO_CURLY_BRACES_UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS, '/default/path:with/colon')
-//   ct.equal(parsed.NO_CURLY_BRACES_UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS2, '/default/path:with/colon')
-//
-//   ct.end()
-// })
-//
-// t.test('should expand with default nested value correctly', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS_NESTED, '/default/path:with/colon')
-//   ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS_NESTED2, '/default/path:with/colon')
-//
-//   ct.end()
-// })
-//
-// t.test('should expand variables with "." in names correctly', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed['POSTGRESQL.MAIN.USER'], parsed['POSTGRESQL.BASE.USER'])
-//
-//   ct.end()
-// })
-//
-// t.test('handles value of only $', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.DOLLAR, '$')
-//
-//   ct.end()
-// })
-//
-// t.test('handles $one$two', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.ONETWO, 'onetwo')
-//   ct.equal(parsed.ONETWO_SIMPLE, 'onetwo')
-//   ct.equal(parsed.ONETWO_SIMPLE2, 'onetwo')
-//   ct.equal(parsed.ONETWO_SUPER_SIMPLE, 'onetwo')
-//
-//   ct.end()
-// })
-//
-// t.test('handles two dollar signs', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.TWO_DOLLAR_SIGNS, 'abcd$')
-//
-//   ct.end()
-// })
-//
-// t.test('does not choke', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.DONT_CHOKE1, '.kZh`>4[,[DDU-*Jt+[;8-,@K=,9%;F9KsoXqOE)gpG^X!{)Q+/9Fc(QF}i[NEi!')
-//   ct.equal(parsed.DONT_CHOKE2, '=;+=CNy3)-D=zI6gRP2w$B@0K;Y]e^EFnCmx$Dx?;.9wf-rgk1BcTR0]JtY<S:b_')
-//   ct.equal(parsed.DONT_CHOKE3, 'MUcKSGSY@HCON<1S_siWTP`DgS*Ug],mu]SkqI|7V2eOk9:>&fw;>HEwms`D8E2H')
-//   ct.equal(parsed.DONT_CHOKE4, 'm]zjzfRItw2gs[2:{p{ugENyFw9m)tH6_VCQzer`*noVaI<vqa3?FZ9+6U;K#Bfd')
-//   ct.equal(parsed.DONT_CHOKE5, '#la__nK?IxNlQ%`5q&DpcZ>Munx=[1-AMgAcwmPkToxTaB?kgdF5y`A8m=Oa-B!)')
-//   ct.equal(parsed.DONT_CHOKE6, 'xlC&*<j4J<d._<JKH0RBJV!4(ZQEN-+&!0p137<g*hdY2H4xk?/;KO1$(W{:Wc}Q')
-//   ct.equal(parsed.DONT_CHOKE7, '?$6)m*xhTVewc#NVVgxX%eBhJjoHYzpXFg=gzn[rWXPLj5UWj@z$/UDm8o79n/p%')
-//   ct.equal(parsed.DONT_CHOKE8, '@}:[4#g%[R-CFR});bY(Z[KcDQDsVn2_y4cSdU<Mjy!c^F`G<!Ks7]kbS]N1:bP:')
-//
-//   ct.end()
-// })
-//
-// t.test('expands self without a recursive call stack error', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.EXPAND_SELF, '$EXPAND_SELF') // because it ends up accessing parsed[key].
-//
-//   ct.end()
-// })
-//
-// t.test('expands DOMAIN with ${HOST}', ct => {
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.HOST, 'something')
-//   ct.equal(parsed.DOMAIN, 'https://something')
-//
-//   ct.end()
-// })
-//
-// t.test('does not attempt to expand password if already existed in processEnv', ct => {
-//   process.env.PASSWORD = 'pas$word'
-//
-//   const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
-//   dotenvExpand.expand(dotenv)
-//
-//   ct.equal(process.env.PASSWORD, 'pas$word')
-//
-//   ct.end()
-// })
-//
-// t.test('does not expand dollar sign that are not variables', ct => {
-//   const dotenv = {
-//     parsed: {
-//       NO_VARIABLES: '\\$.$+$-$$'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.NO_VARIABLES, '$.$+$-$$')
-//
-//   ct.end()
-// })
-//
-// t.test('expands recursively', ct => {
-//   const dotenv = {
-//     parsed: {
-//       MOCK_SERVER_PORT: '8090',
-//       MOCK_SERVER_HOST: 'http://localhost:${MOCK_SERVER_PORT}',
-//       BACKEND_API_HEALTH_CHECK_URL: '${MOCK_SERVER_HOST}/ci-health-check'
-//     }
-//   }
-//   const parsed = dotenvExpand.expand(dotenv).parsed
-//
-//   ct.equal(parsed.MOCK_SERVER_PORT, '8090')
-//   ct.equal(parsed.MOCK_SERVER_HOST, 'http://localhost:8090')
-//   ct.equal(parsed.BACKEND_API_HEALTH_CHECK_URL, 'http://localhost:8090/ci-health-check')
-//
-//   ct.end()
-// })
+t.test('returns object', ct => {
+  const dotenv = { parsed: {} }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  t.ok(parsed instanceof Object, 'should be an object')
+
+  ct.end()
+})
+
+t.test('expands environment variables', ct => {
+  const dotenv = {
+    parsed: {
+      BASIC: 'basic',
+      BASIC_EXPAND: '${BASIC}',
+      BASIC_EXPAND_SIMPLE: '$BASIC'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.BASIC_EXPAND, 'basic')
+  ct.equal(parsed.BASIC_EXPAND_SIMPLE, 'basic')
+
+  ct.end()
+})
+
+t.test('expands self without a recursive call stack error', ct => {
+  const dotenv = {
+    parsed: {
+      EXPAND_SELF: '$EXPAND_SELF'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_SELF, '$EXPAND_SELF') // because it ends up accessing parsed[key].
+
+  ct.end()
+})
+
+t.test('uses environment variables existing already on the machine for expansion', ct => {
+  process.env.MACHINE = 'machine'
+  const dotenv = {
+    parsed: {
+      MACHINE_EXPAND: '${MACHINE}',
+      MACHINE_EXPAND_SIMPLE: '$MACHINE'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MACHINE_EXPAND, 'machine')
+  ct.equal(parsed.MACHINE_EXPAND_SIMPLE, 'machine')
+
+  ct.end()
+})
+
+t.test('does not expand environment variables existing already on the machine that look like they could expand', ct => {
+  process.env.PASSWORD = 'pas$word'
+  const dotenv = {
+    parsed: {
+      PASSWORD: 'dude',
+      PASSWORD_EXPAND: '${PASSWORD}',
+      PASSWORD_EXPAND_SIMPLE: '$PASSWORD'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.PASSWORD_EXPAND, 'pas$word')
+  ct.equal(parsed.PASSWORD_EXPAND_SIMPLE, 'pas$word')
+  ct.equal(parsed.PASSWORD, 'pas$word')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string', ct => {
+  const dotenv = {
+    parsed: {
+      UNDEFINED_EXPAND: '$UNDEFINED'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND, '')
+
+  ct.end()
+})
+
+t.test('prioritizes machine key expansion over .env', ct => {
+  process.env.MACHINE = 'machine'
+  const dotenv = {
+    parsed: {
+      MACHINE: 'machine_env',
+      MACHINE_EXPAND: '$MACHINE'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MACHINE_EXPAND, 'machine')
+
+  ct.end()
+})
+
+t.test('does not expand escaped variables', ct => {
+  const dotenv = {
+    parsed: {
+      ESCAPED_EXPAND: '\\$ESCAPED'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.ESCAPED_EXPAND, '$ESCAPED')
+
+  ct.end()
+})
+
+t.test('does not expand inline escaped dollar sign', ct => {
+  const dotenv = {
+    parsed: {
+      INLINE_ESCAPED_EXPAND: 'pa\\$\\$word'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.INLINE_ESCAPED_EXPAND, 'pa$$word')
+
+  ct.end()
+})
+
+t.test('does not overwrite preset variables', ct => {
+  process.env.SOME_ENV = 'production'
+  const dotenv = {
+    parsed: {
+      SOME_ENV: 'development'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.SOME_ENV, 'production')
+
+  ct.end()
+})
+
+t.test('does not expand inline escaped dollar sign', ct => {
+  const dotenv = {
+    parsed: {
+      INLINE_ESCAPED_EXPAND_BCRYPT: '\\$2b\\$10\\$OMZ69gxxsmRgwAt945WHSujpr/u8ZMx.xwtxWOCMkeMW7p3XqKYca'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.INLINE_ESCAPED_EXPAND_BCRYPT, '$2b$10$OMZ69gxxsmRgwAt945WHSujpr/u8ZMx.xwtxWOCMkeMW7p3XqKYca')
+
+  ct.end()
+})
+
+t.test('handle mixed values', ct => {
+  const dotenv = {
+    parsed: {
+      PARAM1: '42',
+      MIXED_VALUES: '\\$this$PARAM1\\$is${PARAM1}'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MIXED_VALUES, '$this42$is42')
+
+  ct.end()
+})
+
+t.test('expands environment variables (process.env)', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.BASIC_EXPAND, 'basic')
+
+  ct.end()
+})
+
+t.test('expands environment variables (process.env)', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.BASIC_EXPAND, 'basic')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.MACHINE_EXPAND, 'machine')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine (process.env)', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.MACHINE_EXPAND, 'machine')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND, '')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string (process.env)', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND, '')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine even with a default', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.EXPAND_DEFAULT, 'machine')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine even with a default when nested', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.EXPAND_DEFAULT_NESTED, 'machine')
+  ct.equal(process.env.EXPAND_DEFAULT_NESTED2, 'machine')
+
+  ct.end()
+})
+
+t.test('expands environment variables undefined with one already on the machine even with a default when nested', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.UNDEFINED_EXPAND_NESTED, 'machine')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string but replaces with default', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT, 'default')
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT2, 'default')
+
+  ct.end()
+})
+
+t.test('expands environent variables and concats with default nested', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE, 'machine_envdefault')
+  ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE2, 'machine_envdefault')
+
+  ct.end()
+})
+
+t.test('expands environent variables and concats with default nested', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE, 'machine_envdefault')
+  ct.equal(parsed.EXPAND_DEFAULT_NESTED_TWICE2, 'machine_envdefault')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string but replaces with default nested', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED, 'default')
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED2, 'default')
+
+  ct.end()
+})
+
+t.test('expands missing environment variables to an empty string but replaces with default nested twice', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED_TWICE, 'default')
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_NESTED_TWICE2, 'default')
+
+  ct.end()
+})
+
+t.test('prioritizes machine key expansion over .env', ct => {
+  process.env.MACHINE = 'machine'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MACHINE_EXPAND, 'machine')
+
+  ct.end()
+})
+
+t.test('multiple expand', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MONGOLAB_URI, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
+
+  ct.end()
+})
+
+t.test('should expand recursively', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MONGOLAB_URI_RECURSIVELY, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
+
+  ct.end()
+})
+
+t.test('multiple expand', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.NO_CURLY_BRACES_URI, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
+
+  ct.end()
+})
+
+t.test('should expand recursively', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.NO_CURLY_BRACES_URI_RECURSIVELY, 'mongodb://username:password@abcd1234.mongolab.com:12345/heroku_db')
+
+  ct.end()
+})
+
+t.test('can write to an object rather than process.env if user provides it', ct => {
+  const myObject = {}
+  const dotenv = {
+    processEnv: myObject,
+    parsed: {
+      SHOULD_NOT_EXIST: 'testing'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+  const evaluation = typeof process.env.SHOULD_NOT_EXIST
+
+  ct.equal(parsed.SHOULD_NOT_EXIST, 'testing')
+  ct.equal(myObject.SHOULD_NOT_EXIST, 'testing')
+  ct.equal(evaluation, 'undefined')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine even with a default with special characters', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS, 'machine_env')
+  ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS2, 'machine_env')
+
+  ct.end()
+})
+
+t.test('expands environment variables existing already on the machine even with a default with special characters (process.env)', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS, 'machine_env')
+  ct.equal(parsed.EXPAND_DEFAULT_SPECIAL_CHARACTERS2, 'machine_env')
+
+  ct.end()
+})
+
+t.test('should expand with default value correctly', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS, '/default/path:with/colon')
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS2, '/default/path:with/colon')
+  ct.equal(parsed.NO_CURLY_BRACES_UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS, '/default/path:with/colon')
+  ct.equal(parsed.NO_CURLY_BRACES_UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS2, '/default/path:with/colon')
+
+  ct.end()
+})
+
+t.test('should expand with default nested value correctly', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS_NESTED, '/default/path:with/colon')
+  ct.equal(parsed.UNDEFINED_EXPAND_DEFAULT_SPECIAL_CHARACTERS_NESTED2, '/default/path:with/colon')
+
+  ct.end()
+})
+
+t.test('should expand variables with "." in names correctly', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed['POSTGRESQL.MAIN.USER'], parsed['POSTGRESQL.BASE.USER'])
+
+  ct.end()
+})
+
+t.test('handles value of only $', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.DOLLAR, '$')
+
+  ct.end()
+})
+
+t.test('handles $one$two', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.ONETWO, 'onetwo')
+  ct.equal(parsed.ONETWO_SIMPLE, 'onetwo')
+  ct.equal(parsed.ONETWO_SIMPLE2, 'onetwo')
+  ct.equal(parsed.ONETWO_SUPER_SIMPLE, 'onetwo')
+
+  ct.end()
+})
+
+t.test('handles two dollar signs', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.TWO_DOLLAR_SIGNS, 'abcd$')
+
+  ct.end()
+})
+
+t.test('does not choke', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.DONT_CHOKE1, '.kZh`>4[,[DDU-*Jt+[;8-,@K=,9%;F9KsoXqOE)gpG^X!{)Q+/9Fc(QF}i[NEi!')
+  ct.equal(parsed.DONT_CHOKE2, '=;+=CNy3)-D=zI6gRP2w$B@0K;Y]e^EFnCmx$Dx?;.9wf-rgk1BcTR0]JtY<S:b_')
+  ct.equal(parsed.DONT_CHOKE3, 'MUcKSGSY@HCON<1S_siWTP`DgS*Ug],mu]SkqI|7V2eOk9:>&fw;>HEwms`D8E2H')
+  ct.equal(parsed.DONT_CHOKE4, 'm]zjzfRItw2gs[2:{p{ugENyFw9m)tH6_VCQzer`*noVaI<vqa3?FZ9+6U;K#Bfd')
+  ct.equal(parsed.DONT_CHOKE5, '#la__nK?IxNlQ%`5q&DpcZ>Munx=[1-AMgAcwmPkToxTaB?kgdF5y`A8m=Oa-B!)')
+  ct.equal(parsed.DONT_CHOKE6, 'xlC&*<j4J<d._<JKH0RBJV!4(ZQEN-+&!0p137<g*hdY2H4xk?/;KO1$(W{:Wc}Q')
+  ct.equal(parsed.DONT_CHOKE7, '?$6)m*xhTVewc#NVVgxX%eBhJjoHYzpXFg=gzn[rWXPLj5UWj@z$/UDm8o79n/p%')
+  ct.equal(parsed.DONT_CHOKE8, '@}:[4#g%[R-CFR});bY(Z[KcDQDsVn2_y4cSdU<Mjy!c^F`G<!Ks7]kbS]N1:bP:')
+
+  ct.end()
+})
+
+t.test('expands self without a recursive call stack error (process.env)', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test', processEnv: {} })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.EXPAND_SELF, '$EXPAND_SELF') // because it ends up accessing parsed[key].
+
+  ct.end()
+})
+
+t.test('expands DOMAIN with ${HOST}', ct => {
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.HOST, 'something')
+  ct.equal(parsed.DOMAIN, 'https://something')
+
+  ct.end()
+})
+
+t.test('does not attempt to expand password if already existed in processEnv', ct => {
+  process.env.PASSWORD = 'pas$word'
+
+  const dotenv = require('dotenv').config({ path: 'tests/.env.test' })
+  dotenvExpand.expand(dotenv)
+
+  ct.equal(process.env.PASSWORD, 'pas$word')
+
+  ct.end()
+})
+
+t.test('does not expand dollar sign that are not variables', ct => {
+  const dotenv = {
+    parsed: {
+      NO_VARIABLES: '\\$.$+$-$$'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.NO_VARIABLES, '$.$+$-$$')
+
+  ct.end()
+})
+
+t.test('expands recursively', ct => {
+  const dotenv = {
+    parsed: {
+      MOCK_SERVER_PORT: '8090',
+      MOCK_SERVER_HOST: 'http://localhost:${MOCK_SERVER_PORT}',
+      BACKEND_API_HEALTH_CHECK_URL: '${MOCK_SERVER_HOST}/ci-health-check'
+    }
+  }
+  const parsed = dotenvExpand.expand(dotenv).parsed
+
+  ct.equal(parsed.MOCK_SERVER_PORT, '8090')
+  ct.equal(parsed.MOCK_SERVER_HOST, 'http://localhost:8090')
+  ct.equal(parsed.BACKEND_API_HEALTH_CHECK_URL, 'http://localhost:8090/ci-health-check')
+
+  ct.end()
+})
 
 t.test('expands recursively reverse order', ct => {
   const dotenv = {
     parsed: {
       BACKEND_API_HEALTH_CHECK_URL: '${MOCK_SERVER_HOST}/ci-health-check',
       MOCK_SERVER_HOST: 'http://localhost:${MOCK_SERVER_PORT}',
-      // MOCK_SERVER_PORT: '8090'
+      MOCK_SERVER_PORT: '8090'
     }
   }
   const parsed = dotenvExpand.expand(dotenv).parsed