Add testOnly util

Fixes af#32

README.md

@@ -169,6 +169,11 @@ const validatedConfig = envalid.cleanEnv(
 )
 ```
 
+## Utils
+
+### testOnly
+
+A function called `testOnly` is exported. It returns its value if `NODE_ENV === 'test'`, otherwise it returns `undefined`.
 
 ## Motivation
 

index.js

@@ -3,6 +3,12 @@ const { EnvError, EnvMissingError, makeValidator,
 
 const extend = (x = {}, y = {}) => Object.assign({}, x, y)
 
+function testOnly(testDefault) {
+    return process.env.NODE_ENV === 'test'
+        ? testDefault
+        : undefined
+}
+
 /**
 * Validate a single env var, given a spec object
 *
@@ -41,8 +47,8 @@ function extendWithDotEnv(inputEnv, dotEnvPath = '.env') {
     // The react-native packager detects the require calls even if they
     // are not on the top level, so we need to hide them by concatinating
     // the module names.
-    const fs = require('f'+'s')
-    const dotenv = require('doten'+'v')
+    const fs = require('f' + 's')
+    const dotenv = require('doten' + 'v')
 
     let dotEnvBuffer = null
     try {
@@ -121,5 +127,5 @@ function cleanEnv(inputEnv, specs = {}, options = {}) {
 }
 
 module.exports = {
-    cleanEnv, makeValidator, bool, num, str, json, url, email, EnvError, EnvMissingError
+    cleanEnv, makeValidator, bool, num, str, json, url, email, EnvError, EnvMissingError, testOnly
 }

tests/test_basics.js

@@ -1,5 +1,5 @@
 const { createGroup, assert } = require('painless')
-const { cleanEnv, EnvError, EnvMissingError, str, num } = require('..')
+const { cleanEnv, EnvError, EnvMissingError, str, num, testOnly } = require('..')
 const { assertPassthrough } = require('./utils')
 const test = createGroup()
 const makeSilent = { reporter: null }
@@ -146,3 +146,22 @@ test('NODE_ENV built-in support', () => {
     assert.strictEqual(cleanEnv({}, customSpec).isProduction, false)
     assert.strictEqual(cleanEnv({}, customSpec).isDev, false)
 })
+
+test('testOnly', () => {
+    const processEnv = process.env.NODE_ENV
+    process.env.NODE_ENV = 'test'
+    let spec = {
+        FOO: str({ devDefault: testOnly('sup') })
+    }
+
+    process.env.NODE_ENV = processEnv
+    const env = cleanEnv({ NODE_ENV: 'test' }, spec)
+    assert.deepEqual(env, { NODE_ENV: 'test', FOO: 'sup' })
+
+    spec = {
+        FOO: str({ devDefault: testOnly('sup') })
+    }
+
+    assert.throws(() => cleanEnv({ NODE_ENV: 'production' }, spec, makeSilent), EnvMissingError)
+    assert.throws(() => cleanEnv({ NODE_ENV: 'development' }, spec, makeSilent), EnvMissingError)
+})