Glossary — L
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Latency
The time it takes for a single round-trip request between a client and a server. Measured in milliseconds. “Round-trip” means the time from “I sent it” to “I got an answer back.” Geographic distance, network congestion, and server speed all add to latency. The whole point of CDNs and edge networks is to reduce it.
Library
A collection of reusable code you can drop into your project. React is a library, Lodash is a library, date-fns is a library. Contrast with a framework — a library you call; a framework calls you.
Linting
Automated checking of your code for style issues and common bugs. ESLint is the standard for JavaScript/TypeScript. Lint rules can catch things like unused variables, missing dependencies in React hooks, or accidentally using == instead of ===. Lint errors block your CI build if configured to.
See also: Linting
Linux
The open-source operating system that powers nearly every server on the internet — including the ones your Next.js app runs on after deployment. You don’t need to know how to use Linux directly to develop on Windows or macOS, but understanding that “production runs Linux” explains a lot of behaviour (case-sensitive filenames, certain shell expectations).
See also: Operating systems
LLM (Large Language Model)
A neural network trained on enormous amounts of text to predict the next token, given a sequence of tokens. By doing this at huge scale (hundreds of billions of parameters), LLMs end up able to write essays, code, summaries, explanations, and so on. Claude, GPT-4, Gemini, and Llama are all LLMs.
See also: What is an LLM?, Tokens
Local storage
A browser-built-in key-value store, persistent across sessions, accessible via JavaScript:
localStorage.setItem("theme", "dark");
const theme = localStorage.getItem("theme");Holds up to ~5MB per domain. Plain text — don’t store secrets. Vulnerable to XSS — if attacker JS runs on your page, it can read everything in localStorage. For auth tokens, HTTP-only cookies are usually safer.
Lock file
A file (package-lock.json for npm, pnpm-lock.yaml for pnpm, yarn.lock for Yarn) that records the exact versions of every dependency — direct and transitive — installed in your project. Commit this to Git. Without it, two developers running npm install at different times could end up with different code in node_modules/.
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