fix: Honor eager setting (false by default) (#1179)

This delays loading of istanbul-lib-instrument until it is needed.

lib/instrumenters/istanbul.js

@@ -1,10 +1,10 @@
 'use strict'
 
-const { createInstrumenter } = require('istanbul-lib-instrument')
-const convertSourceMap = require('convert-source-map')
-const mergeSourceMap = require('merge-source-map')
-
 function InstrumenterIstanbul (options) {
+  const { createInstrumenter } = require('istanbul-lib-instrument')
+  const convertSourceMap = require('convert-source-map')
+  const mergeSourceMap = require('merge-source-map')
+
   const { plugins } = options
   const configPlugins = plugins ? { plugins } : {}
 
@@ -28,6 +28,7 @@ function InstrumenterIstanbul (options) {
       // TODO: test source-map merging logic.
       if (options.produceSourceMap) {
         var lastSourceMap = instrumenter.lastSourceMap()
+        /* istanbul ignore else */
         if (lastSourceMap) {
           if (sourceMap) {
             lastSourceMap = mergeSourceMap(

lib/instrumenters/noop.js

@@ -1,6 +1,8 @@
-const { readInitialCoverage } = require('istanbul-lib-instrument')
+'use strict'
 
 function NOOP () {
+  const { readInitialCoverage } = require('istanbul-lib-instrument')
+
   return {
     instrumentSync (code, filename) {
       const extracted = readInitialCoverage(code)

tap-snapshots/test-eager.js-TAP.test.js

@@ -0,0 +1,16 @@
+/* IMPORTANT
+ * This snapshot file is auto-generated, but designed for humans.
+ * It should be checked into source control and tracked carefully.
+ * Re-generate by setting TAP_SNAPSHOT=1 and running tests.
+ * Make sure to inspect the output below.  Do not ignore changes!
+ */
+'use strict'
+exports[`test/eager.js TAP eager disabled by default > stdout 1`] = `
+0
+
+`
+
+exports[`test/eager.js TAP eager enabled > stdout 1`] = `
+1
+
+`

test/eager.js

@@ -0,0 +1,30 @@
+'use strict'
+
+const path = require('path')
+
+const t = require('tap')
+
+const { testSuccess } = require('./helpers')
+
+const cwd = path.resolve(__dirname, 'fixtures')
+
+t.test('eager disabled by default', t => testSuccess(t, {
+  args: [
+    '--silent=true',
+    '--exclude=eager.js',
+    process.execPath,
+    './eager.js'
+  ],
+  cwd
+}))
+
+t.test('eager enabled', t => testSuccess(t, {
+  args: [
+    '--silent=true',
+    '--eager=true',
+    '--exclude=eager.js',
+    process.execPath,
+    './eager.js'
+  ],
+  cwd
+}))

test/fixtures/eager.js

@@ -0,0 +1,5 @@
+#!/usr/bin/env node
+'use strict'
+
+const lib = require.resolve('istanbul-lib-instrument')
+console.log(Object.keys(require.cache).filter(s => s === lib).length)