https://learn-typescript.org/ Free TypeScript tutorials, plus how to use the type system as a correctness harness for AI-generated code. Sun, 06 Sep 2026 03:07:12 GMT https://learn-typescript.org/hello-world/https://learn-typescript.org/hello-world/Your first TypeScript file, and the two commands that check it and run it. https://learn-typescript.org/ai/types-as-harness/https://learn-typescript.org/ai/types-as-harness/A type is a specification the compiler enforces on every edit, for free, forever. That makes TypeScript unusually well suited to a workflow where a machine writes most of the code. https://learn-typescript.org/review/failure-modes/https://learn-typescript.org/review/failure-modes/The compiler catches a lot. What survives is almost always the compiler being told to look away. https://learn-typescript.org/variables-and-types/https://learn-typescript.org/variables-and-types/TypeScript is JavaScript plus a type checker. This is where the types start — and where you stop writing var. https://learn-typescript.org/arrays/https://learn-typescript.org/arrays/TypeScript arrays are JavaScript arrays with an element type — `string[]` rather than a bag of anything. https://learn-typescript.org/review/dependencies/https://learn-typescript.org/review/dependencies/Everything npm does wrong, plus a second parallel dependency graph made of type definitions that nobody reviews. https://learn-typescript.org/ai/agents-md/https://learn-typescript.org/ai/agents-md/Half of what people put in a TypeScript instructions file belongs in tsconfig.json instead, where it is enforced rather than suggested. https://learn-typescript.org/manipulating-arrays/https://learn-typescript.org/manipulating-arrays/Arrays can also function as a stack. The push and pop methods insert and remove variables from the end of an array. https://learn-typescript.org/review/security/https://learn-typescript.org/review/security/Types are erased at runtime, so an attacker never sees them. The security value of TypeScript is real but indirect — and generated code reaches for the escape hatches at exactly the wrong moments. https://learn-typescript.org/ai/evals/https://learn-typescript.org/ai/evals/A model returns a string. Your application needs a value. Everything interesting about building LLM apps in TypeScript happens at that boundary — and it is the one place `as` will hurt you most. https://learn-typescript.org/review/performance/https://learn-typescript.org/review/performance/Runtime performance is JavaScript's problem and is covered there. TypeScript's own performance problem is the compiler — and a slow one directly degrades the quality of everything an agent writes. https://learn-typescript.org/ai/tokenomics/https://learn-typescript.org/ai/tokenomics/You cannot type-check a bill. You can make it impossible to call a model without a budget, impossible to confuse dollars with tokens, and impossible to add a call site nobody is measuring. https://learn-typescript.org/operators/https://learn-typescript.org/operators/The arithmetic, comparison and logical operators, and the two TypeScript-relevant ones: ?? and ?. https://learn-typescript.org/conditions/https://learn-typescript.org/conditions/The if statement allows us to check if an expression is equal to true or false, and execute different code according to the result. https://learn-typescript.org/loops/https://learn-typescript.org/loops/for, for...of, for...in and while — and which one to reach for. https://learn-typescript.org/objects/https://learn-typescript.org/objects/Objects hold keyed values, and in TypeScript the set of keys and their types is part of the type. https://learn-typescript.org/functions/https://learn-typescript.org/functions/Functions are code blocks that can have arguments, and function have their own scope. https://learn-typescript.org/pop-up-boxes/https://learn-typescript.org/pop-up-boxes/There are three types of pop-up boxes in javascript: confirm, alert, and prompt. https://learn-typescript.org/callbacks/https://learn-typescript.org/callbacks/Passing a function to another function — and typing its signature so the parameters come out inferred. https://learn-typescript.org/arrow-functions/https://learn-typescript.org/arrow-functions/Arrow functions are a feature of ES6, their behavior are generally the same of a function. https://learn-typescript.org/object-oriented-javascript/https://learn-typescript.org/object-oriented-javascript/JavaScript uses functions as classes to create objects using the new keyword. https://learn-typescript.org/function-context/https://learn-typescript.org/function-context/Functions in JavaScript run in a specific context, and using the this variable we have access to it. https://learn-typescript.org/inheritance/https://learn-typescript.org/inheritance/JavaScript uses prototype based inheritance. https://learn-typescript.org/destructuring/https://learn-typescript.org/destructuring/Destructuring is a feature of ES6, introduced for making easier and cleaner some repetitive operations and assignments made in JS. https://learn-typescript.org/functions-and-signatures/https://learn-typescript.org/functions-and-signatures/A function signature is the most valuable annotation you will write — it is a contract the compiler enforces at every call site. https://learn-typescript.org/interfaces-and-type-aliases/https://learn-typescript.org/interfaces-and-type-aliases/Naming the shape of your data is the point at which TypeScript starts paying for itself. https://learn-typescript.org/union-types-and-narrowing/https://learn-typescript.org/union-types-and-narrowing/A union says a value is one of several things. Narrowing is how you prove which one — and it is the most distinctively TypeScript skill there is. https://learn-typescript.org/generics/https://learn-typescript.org/generics/A generic is a type with a hole in it. They look intimidating and the everyday use is genuinely simple. https://learn-typescript.org/classes-and-modifiers/https://learn-typescript.org/classes-and-modifiers/TypeScript adds real access control, parameter properties and interface implementation to JavaScript classes. https://learn-typescript.org/async-and-promises/https://learn-typescript.org/async-and-promises/Typing asynchronous code, and the lint rule that catches the most common bug in TypeScript and JavaScript alike.