diff --git a/package-lock.json b/package-lock.json
index 933ef39..37e3d09 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -190,6 +190,12 @@
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@@ -1734,6 +1793,12 @@
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@@ -1769,6 +1834,16 @@
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+      "integrity": "sha1-Sb1jMdfQLQwJvJEKEHW6gWW1bfk=",
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@@ -2127,6 +2202,41 @@
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+      "resolved": "https://registry.npmjs.org/mocha/-/mocha-4.0.1.tgz",
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+      "dev": true,
+      "requires": {
+        "browser-stdout": "1.3.0",
+        "commander": "2.11.0",
+        "debug": "3.1.0",
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+        "escape-string-regexp": "1.0.5",
+        "glob": "7.1.2",
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+        "he": "1.1.1",
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+          "dev": true,
+          "requires": {
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+      "integrity": "sha1-VgiurfwAvmwpAd9fmGF4jeDVl8g=",
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@@ -2225,6 +2335,15 @@
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+      "integrity": "sha1-WDsap3WWHUsROsF9nFC6753Xa9E=",
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@@ -2646,6 +2765,60 @@
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+      "resolved": "https://registry.npmjs.org/should/-/should-13.1.3.tgz",
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+      "dev": true,
+      "requires": {
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+        "should-format": "3.0.3",
+        "should-type": "1.4.0",
+        "should-type-adaptors": "1.0.1",
+        "should-util": "1.0.0"
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+    "should-equal": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/should-equal/-/should-equal-2.0.0.tgz",
+      "integrity": "sha512-ZP36TMrK9euEuWQYBig9W55WPC7uo37qzAEmbjHz4gfyuXrEUgF8cUvQVO+w+d3OMfPvSRQJ22lSm8MQJ43LTA==",
+      "dev": true,
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+      "resolved": "https://registry.npmjs.org/should-format/-/should-format-3.0.3.tgz",
+      "integrity": "sha1-m/yPdPo5IFxT04w01xcwPidxJPE=",
+      "dev": true,
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+      "resolved": "https://registry.npmjs.org/should-type/-/should-type-1.4.0.tgz",
+      "integrity": "sha1-B1bYzoRt/QmEOmlHcZ36DUz/XPM=",
+      "dev": true
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+    "should-type-adaptors": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/should-type-adaptors/-/should-type-adaptors-1.0.1.tgz",
+      "integrity": "sha1-7+VVPN9oz/ZuXF9RtxLcNRx3vqo=",
+      "dev": true,
+      "requires": {
+        "should-type": "1.4.0",
+        "should-util": "1.0.0"
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+      "resolved": "https://registry.npmjs.org/should-util/-/should-util-1.0.0.tgz",
+      "integrity": "sha1-yYzaN0qmsZDfi6h8mInCtNtiAGM=",
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@@ -3019,6 +3192,12 @@
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+      "resolved": "https://registry.npmjs.org/wrappy/-/wrappy-1.0.2.tgz",
+      "integrity": "sha1-tSQ9jz7BqjXxNkYFvA0QNuMKtp8=",
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diff --git a/package.json b/package.json
index 4651797..ec1a623 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,8 @@
   },
   "homepage": "https://github.com/dmuth/diceware#readme",
   "devDependencies": {
+    "mocha": "^4.0.1",
+    "should": "^13.1.3",
     "webpack": "^3.8.1"
   },
   "dependencies": {