SWE Interview Flashcards
(30 cards)
What is the difference between var, let, and const in JavaScript?
var is function-scoped and can be redeclared; let is block-scoped and reassignable; const is block-scoped and cannot be reassigned, but objects can be mutated.
How does this work in JavaScript?
this refers to the context of a function’s call. In global scope, it’s window (browser). In a method, it’s the object. Arrow functions inherit this lexically.
What is a closure in JavaScript?
A closure is a function that retains access to its outer scope’s variables. Example: function outer() { let x = 10; return () => x; } returns a function accessing x.
What is event delegation in JavaScript?
Event delegation handles events on a parent element for its children, using event bubbling. Example: Add click listener to a ul to handle li clicks (reduces listeners).
Explain the event loop in JavaScript.
The event loop processes the call stack and callback queue, handling asynchronous tasks (e.g., setTimeout, promises) when the stack is empty, enabling non-blocking I/O.
What is a Promise in JavaScript?
A Promise represents an asynchronous operation’s outcome (pending, fulfilled, rejected). Example: new Promise((resolve, reject) => resolve('Done')).
How do async and await simplify asynchronous code?
async functions return Promises; await pauses execution until a Promise resolves. Example: async function fetchData() { let data = await fetch(url); }.
What is a React component?
A React component is a function or class returning UI as JSX. Example: function MyComponent() { return <div>Hello</div>; }.
What is the purpose of useState in React?
useState manages state in functional components, returning a state value and updater. Example: const [count, setCount] = useState(0);.
Explain the useEffect hook in React.
useEffect runs side effects (e.g., API calls) after render, with a dependency array to control execution. Example: useEffect(() => { fetchData(); }, [id]);.
What is a controlled component in React?
A controlled component ties form input values to React state, updated via onChange. Example: <input value={state} onChange={(e) => setState(e.target.value)} />.
How does React.memo improve performance?
React.memo memoizes a functional component, preventing re-renders unless props change. Example: const Memoized = React.memo(MyComponent);.
What is the virtual DOM in React?
The virtual DOM is an in-memory representation of the real DOM. React diffs it with the previous version to minimize DOM updates.
What is a data structure?
A data structure organizes data for efficient access/modification, like arrays (contiguous memory) or linked lists (nodes with pointers).
What is an algorithm?
An algorithm is a step-by-step procedure to solve a problem, like sorting an array or searching for an item.
What is Big O notation?
Big O describes the worst-case growth rate of an algorithm’s time or space usage as input size increases. Example: O(n) for linear time.
What is O(log n) time complexity?
O(log n) is logarithmic time, where runtime grows slowly by halving the problem. Example: Binary search on a sorted array.
What is the time complexity of accessing a hash table?
O(1) average case for hash table lookups, assuming minimal collisions and a good hash function.
What is the Sliding Window pattern, with an example?
Sliding Window optimizes subarray/substring problems. Example: function maxSum(arr, k) { let sum = 0, max = 0; for (let i = 0; i < k; i++) sum += arr[i]; max = sum; for (let i = k; i < arr.length; i++) { sum += arr[i] - arr[i-k]; max = Math.max(max, sum); } return max; } (O(n)).
Implement Two Pointers to reverse an array in place.
```javascript
function reverseArray(arr) {
let left = 0, right = arr.length - 1;
while (left < right) {
[arr[left], arr[right]] = [arr[right], arr[left]];
left++;
right–;
}
return arr;
}
// Example: reverseArray([1, 2, 3]) → [3, 2, 1]
// Explanation: Swaps elements from both ends (O(n)).
~~~
Use Binary Search to find the first occurrence of a target.
```javascript
function firstOccurrence(arr, target) {
let left = 0, right = arr.length - 1, result = -1;
while (left <= right) {
let mid = Math.floor((left + right) / 2);
if (arr[mid] === target) {
result = mid;
right = mid - 1;
} else if (arr[mid] < target) {
left = mid + 1;
} else {
right = mid - 1;
}
}
return result;
}
// Example: firstOccurrence([1, 2, 2, 2, 3], 2) → 1
// Explanation: Tracks first occurrence while searching (O(log n)).
~~~
What is Kadane’s Algorithm for max subarray sum?
```javascript
function maxSubArray(nums) {
let max = nums[0], current = nums[0];
for (let i = 1; i < nums.length; i++) {
current = Math.max(nums[i], current + nums[i]);
max = Math.max(max, current);
}
return max;
}
// Example: maxSubArray([-2, 1, -3, 4, -1, 2]) → 6
// Explanation: Tracks local and global max sum (O(n)).
~~~
Explain Depth-First Search (DFS) with a code snippet.
```javascript
function dfs(graph, start, visited = new Set()) {
visited.add(start);
console.log(start);
for (let neighbor of graph[start]) {
if (!visited.has(neighbor)) dfs(graph, neighbor, visited);
}
}
// Example: graph = {0:[1,2], 1:[2], 2:[]} → dfs(graph, 0) prints 0,1,2
// Explanation: Recursively explores each branch (O(V + E)).
~~~
What is the Merge Intervals pattern?
```javascript
function mergeIntervals(intervals) {
intervals.sort((a, b) => a[0] - b[0]);
let result = [intervals[0]];
for (let i = 1; i < intervals.length; i++) {
let last = result[result.length - 1];
if (intervals[i][0] <= last[1]) {
last[1] = Math.max(last[1], intervals[i][1]);
} else {
result.push(intervals[i]);
}
}
return result;
}
// Example: mergeIntervals([[1,3], [2,6], [8,10]]) → [[1,6], [8,10]]
// Explanation: Merges overlapping intervals (O(n log n)).
~~~
