Warn when functions intended as constructors are called without new (#2021)

* Warn when pg.Pool() isn’t called as a constructor

in preparation for #1541. `eval` is a bit awkward, but it’s more accurate than an `instanceof` check and will work on platforms new enough to support pg 8 (i.e. only not Node 4).

* Warn when Query() isn’t called as a constructor

lib/compat/check-constructor.js

@@ -0,0 +1,22 @@
+'use strict'
+
+const warnDeprecation = require('./warn-deprecation')
+
+// Node 4 doesn’t support new.target.
+let hasNewTarget
+
+try {
+  // eslint-disable-next-line no-eval
+  eval('(function () { new.target })')
+  hasNewTarget = true
+} catch (error) {
+  hasNewTarget = false
+}
+
+const checkConstructor = (name, code, getNewTarget) => {
+  if (hasNewTarget && getNewTarget() === undefined) {
+    warnDeprecation(`Constructing a ${name} without new is deprecated and will stop working in pg 8.`, code)
+  }
+}
+
+module.exports = checkConstructor

lib/compat/warn-deprecation.js

@@ -0,0 +1,19 @@
+'use strict'
+
+const util = require('util')
+
+const dummyFunctions = new Map()
+
+// Node 4 doesn’t support process.emitWarning(message, 'DeprecationWarning', code).
+const emitDeprecationWarning = (message, code) => {
+  let dummy = dummyFunctions.get(code)
+
+  if (dummy === undefined) {
+    dummy = util.deprecate(() => {}, message)
+    dummyFunctions.set(code, dummy)
+  }
+
+  dummy()
+}
+
+module.exports = emitDeprecationWarning

lib/index.js

@@ -12,9 +12,13 @@ var Client = require('./client')
 var defaults = require('./defaults')
 var Connection = require('./connection')
 var Pool = require('pg-pool')
+const checkConstructor = require('./compat/check-constructor')
 
 const poolFactory = (Client) => {
   var BoundPool = function (options) {
+    // eslint-disable-next-line no-eval
+    checkConstructor('pg.Pool', 'PG-POOL-NEW', () => eval('new.target'))
+
     var config = Object.assign({ Client: Client }, options)
     return new Pool(config)
   }

lib/query.js

@@ -9,12 +9,15 @@
 
 var EventEmitter = require('events').EventEmitter
 var util = require('util')
+const checkConstructor = require('./compat/check-constructor')
 
 var Result = require('./result')
 var utils = require('./utils')
 
 var Query = function (config, values, callback) {
-  // use of "new" optional
+  // use of "new" optional in pg 7
+  // eslint-disable-next-line no-eval
+  checkConstructor('Query', 'PG-QUERY-NEW', () => eval('new.target'))
   if (!(this instanceof Query)) { return new Query(config, values, callback) }
 
   config = utils.normalizeQueryConfig(config, values, callback)