predict

Say what it does, before it does it

A program, unrun, and the questions it chose. Answer them, and then the machine runs it and shows you exactly where you and it disagreed. Nothing here asks you to write code.

• Objects, references and mutation · warm-upTwo names, and a push through one of them.
• Objects, references and mutation · routineA spread, and a change to something inside it.
• Objects, references and mutation · routineA sort, and the array it was called on.
• Hoisting, scope and the dead zone · warm-upTwo declarations, read before their own lines.
• Hoisting, scope and the dead zone · routineA read above a `let`.
• Closures · warm-upA function made before the value changed.
• Closures · routineTwo counters from one factory.
• `this` and prototypes · routineA method, pulled off its object.
• `this` and prototypes · hardTwo objects sharing a prototype, and a write to one of them.
• Control and failure · hardA `return` in a `try`, and another in the `finally`.
• Control and failure · routineA throw, three frames deep.
• Control and failure · routineA `switch` with one missing `break`.
• Reading what was generated · hardA model wrote this helper. Say what it returns and what it changed on the way, before you run it.
• The event loop · routineA model wrote this to read a total. Say what `total` holds when the last line runs.
• The event loop · routineFour lines that log, and two queues.
• Objects, references and mutation · hardWrite `withTag`, which returns a list with the tag added. First say what the reference does to its argument.