Mid-Level Developer 路 2-5 years
Mid-level interviews focus on trade-offs and ownership. Interviewers want to see that you have hit real problems such as slow queries, race conditions or re-render storms, and know how to reason your way through them.
104 questions
Hooks are functions such as useState and useEffect that let function components use state, lifecycle behaviour and other React features; they must be called at the top level of a component or custom hook, never inside loops, conditions or nested functions.
Keys give each list item a stable identity so React can match items between renders and correctly add, remove or reorder them without recreating or mixing up their state.
em is relative to the parent font size, rem is relative to the root font size, % is relative to the parent鈥檚 relevant size, and vh/vw are 1% of the viewport height or width.
Cookies are sent to the server with every matching request and can expire; localStorage persists until cleared and stays in the browser; sessionStorage lasts for the current tab only.
require is the CommonJS module system, which loads modules synchronously at runtime, while import is the ES module standard, which is statically analysed, asynchronous and supports top-level await and tree shaking.
Middleware are functions that receive the request, response and a next callback; they run in the order they are registered and can modify the request, end the response or pass control on with next().
UNION combines two result sets and removes duplicate rows, while UNION ALL combines them and keeps every row, which makes it faster.
A const is a compile-time constant that must be assigned where it is declared, while a readonly field can be assigned at declaration or in the constructor, so its value can be decided at runtime.
Both pass an argument by reference, but ref requires the variable to be initialised before the call, while out requires the method to assign a value before it returns.
*args collects extra positional arguments into a tuple and **kwargs collects extra keyword arguments into a dictionary, which lets a function accept a variable number of arguments.
Vertical scaling means giving one machine more CPU, memory or disk, while horizontal scaling means adding more machines and spreading the load across them.
Volumes are storage managed by Docker that lives outside a container鈥檚 writable layer, so data such as database files survives when the container is removed or replaced.
The test pyramid is a guideline to have many fast unit tests at the base, fewer integration tests in the middle and only a small number of slow end-to-end tests at the top, so the suite stays fast and reliable.
Walk through the array once while storing each number鈥檚 index in a hash map; for every number, check whether target minus that number is already in the map. This runs in O(n) time and O(n) space.
A closure is a function that keeps access to the variables of the scope where it was created, even after that outer function has finished running.
this is decided by how a function is called, not where it is written: as a method it is the object before the dot, with new it is the new object, and arrow functions inherit it from the enclosing scope.
async/await is syntax built on top of promises that lets you write asynchronous code that reads like synchronous code; under the hood every async function returns a promise.
Event delegation means attaching one listener to a parent element and using event bubbling to handle events from its children, instead of adding a listener to each child.
useEffect runs a function after the component renders; the dependency array tells React when to re-run it: with no array it runs after every render, with an empty array only after the first, and with values only when those values change.
A controlled component keeps the input's value in React state and updates it through onChange, while an uncontrolled component lets the DOM hold the value and you read it with a ref when needed.