Install type definitions for Jest

Author: wimpyprogrammer

package.json

@@ -38,6 +38,7 @@
     "@babel/core": "^7.0.0-beta.54",
     "@babel/preset-env": "^7.0.0-beta.54",
     "@babel/preset-typescript": "^7.0.0-beta.54",
+    "@types/jest": "^23.3.1",
     "husky": "^0.14.3",
     "jest": "^23.4.1",
     "prettier": "^1.13.7",

yarn.lock

@@ -543,6 +543,10 @@
     lodash "^4.17.5"
     to-fast-properties "^2.0.0"
 
+"@types/jest@^23.3.1":
+  version "23.3.1"
+  resolved "https://registry.yarnpkg.com/@types/jest/-/jest-23.3.1.tgz#a4319aedb071d478e6f407d1c4578ec8156829cf"
+
 abab@^1.0.4:
   version "1.0.4"
   resolved "https://registry.yarnpkg.com/abab/-/abab-1.0.4.tgz#5faad9c2c07f60dd76770f71cf025b62a63cfd4e"