# Basics AJS is a subset of JavaScript for agent code: programs written by people or by language models that a host runs safely. An **agent** is one AJS function. The **host**, the program that embeds the AJS VM, calls it with an object of inputs, runs it under a fuel budget with only the capabilities it chooses to grant (such as `fetch`), and gets an object back. These seven short pages are the tutorial for the language itself, one idea per page. They assume you know basic JavaScript. For the full list of rules in one place, see [the language guide](/DOCS-AJS/). - [Hello World](/hello-world/): the shape of an agent, a function that takes one object of inputs with defaults and returns one object. - [Math Operations](/math-operations/): arithmetic, `Math`, and turning text into numbers. - [Conditional Logic](/conditional-logic/): `if`, the ternary, lookups instead of `switch`, and falsy values. - [Loop & Filter](/loop-filter/): `for...of`, `while`, and the array methods that take a callback. - [String Processing](/string-processing/): the string methods. - [JSON Processing](/json-processing/): parsing text that may not be JSON, and checking its shape. - [Error Handling](/ajs-error-handling/): errors are values, not thrown exceptions, and which ones a `catch` can take. AJS looks like JavaScript, but a few things behave differently and some are refused. Reading a member of `null` returns `undefined` instead of throwing, and `'total: ' + [1, 2]` is an error instead of the string `total: 1,2`. Each page says where AJS differs from the JavaScript you know for its topic, because those differences are what trip people up. Every agent in these pages runs live: it is editable, and the page transpiles it and runs it on the same sandboxed VM you would embed, with no arguments and a budget of 10,000 fuel. [AJS Examples](/ajs-examples/) describes exactly how. None of these agents needs a network or a model. Read them in order. After the last one you can write an agent that takes input, computes, loops, returns an error value instead of failing on bad input, and returns a result a host can use. If you are embedding the VM, read these to learn what your agents will look like, then see [Embedding the AJS VM](/ajs-host/) and [Capabilities and Custom Atoms](/ajs-capabilities/). Calling the outside world from an agent comes next, in [APIs](/ajs-api/) and [LLMs](/ajs-llm/). - [Hello World](/hello-world/) - [Math Operations](/math-operations/) - [Conditional Logic](/conditional-logic/) - [Loop & Filter](/loop-filter/) - [String Processing](/string-processing/) - [JSON Processing](/json-processing/) - [Error Handling](/ajs-error-handling/)