Defensive JavaScript (DJS) is a typed subset of JavaScript that guarantees that the functional behavior of a program cannot be tampered with even if it is loaded by and executed within a malicious environment under the control of the attacker. As such, DJS is ideal for writing JavaScript security components, such as bookmarklets, single sign-on widgets, and cryptographic libraries, that may be loaded within untrusted web pages alongside unknown scripts from arbitrary third parties. We present a tutorial of the DJS language along with motivations for its design. We show how to program security components in DJS, how to verify their defensiveness using the DJS typechecker, and how to analyze their security properties automatically using ProVerif.