TYPESCRIPT CONFIDENCE LAB
Turn JavaScript knowledge into safer code. Repair real type errors, explore what the compiler knows, and build a contract for data you can actually trust.
// A small contract. A useful catch. let quantity: number = "three"; // Make the data match the intent. let quantity: number = 3;
Confidence comes from understanding
what the check can—and cannot—prove.
REAL TYPESCRIPT. VISIBLE FEEDBACK.
Edit the source and ask the compiler what it finds. This single-file workspace checks types and emits JavaScript; it never executes your code. Third-party imports and JSX are outside this lab’s scope.
Use the keyboard normally; Tab moves to the next control. Your draft is saved in this browser.
Bundled TypeScript 5.9.3. The first check loads the compiler and standard type libraries from this site. A background worker keeps checking off the page’s main thread; a time limit stops unusually expensive checks.
Shows top-level variables and functions, not every intermediate expression.
Check a valid example to see the output.
Output is withheld when the compiler reports errors. Passing type checks does not prove runtime behavior is correct.
MAKE A USEFUL REPAIR
Load a mission into the workshop, edit the code and check the type contract. Missions use strict and indexed-access checking. Passing verifies these type relationships; it does not execute the function or prove its behavior.
Read it, explain it, then try writing the repair yourself.
FEWER POSSIBILITIES. MORE USEFUL OPERATIONS.
This visual model starts with string | number | null | User | string[]. Choose a guard, then test a concrete sample. It illustrates branch logic; use the compiler workshop for actual inferred types.
TYPES DESCRIBE. VALIDATORS CHECK.
Configure a User contract, edit a JSON response and validate it. Nothing is sent to an API. This builder checks flat fields, primitive values, finite numbers and arrays of strings; it does not validate email format or nested objects.
The validator returns errors; it does not silently coerce values or automatically narrow a caller’s variable. Extra fields are allowed unless you choose to reject them.
FROM JAVASCRIPT TO CLEARER CONTRACTS