fix: set proxy from env vars based on truthiness

This matches the behavior of previous versions of
the npm agent. We can't use default parameters since
those will only evaluate if 'undefined' is passed in.

Fixes npm/cli#6835

lib/proxy.js

@@ -64,14 +64,17 @@ const isNoProxy = (url, noProxy) => {
   })
 }
 
-const getProxy = (url, {
-  proxy = PROXY_ENV.https_proxy,
-  noProxy = PROXY_ENV.no_proxy,
-}) => {
+const getProxy = (url, { proxy, noProxy }) => {
   url = urlify(url)
 
-  if (!proxy && url.protocol !== 'https:') {
-    proxy = PROXY_ENV.http_proxy || PROXY_ENV.proxy
+  if (!proxy) {
+    proxy = url.protocol === 'https:'
+      ? PROXY_ENV.https_proxy
+      : PROXY_ENV.https_proxy || PROXY_ENV.http_proxy || PROXY_ENV.proxy
+  }
+
+  if (!noProxy) {
+    noProxy = PROXY_ENV.no_proxy
   }
 
   if (!proxy || isNoProxy(url, noProxy)) {

test/index.js

@@ -140,6 +140,29 @@ t.test('getAgent', (t) => {
     t.end()
   })
 
+  t.test('respects proxy for http target if proxy=null is passed in', (t) => {
+    process.env.proxy = 'http://localhost'
+    t.teardown(() => {
+      delete process.env.proxy
+    })
+
+    const { getAgent: getEnvAgent, HttpAgent: EnvHttpAgent } = t.mock('../lib/index.js')
+
+    const agent = getEnvAgent('http://localhost')
+    t.type(agent, EnvHttpAgent)
+    t.hasStrict(agent, {
+      proxy: {
+        url: {
+          protocol: 'http:',
+          hostname: 'localhost',
+          port: '',
+        },
+      },
+    })
+
+    t.end()
+  })
+
   t.test('ignores http_proxy for https target', (t) => {
     process.env.http_proxy = 'http://localhost'
     t.teardown(() => {
@@ -148,7 +171,7 @@ t.test('getAgent', (t) => {
 
     const { getAgent: getEnvAgent, HttpsAgent: EnvHttpsAgent } = t.mock('../lib/index.js')
 
-    const agent = getEnvAgent('https://localhost')
+    const agent = getEnvAgent('https://localhost', { proxy: null })
     t.type(agent, EnvHttpsAgent)
     t.notOk(agent.proxy.url)