How do stacks and queues work?

The difference is which end you remove from. A stack removes the most recent item, which fits nested structures such as function calls, undo history and bracket matching. A queue removes the oldest item, which fits breadth-first search, scheduling and buffering.

const stack = [];
stack.push("a");
stack.push("b");
const top = stack.pop();

const queue = [];
let head = 0;
queue.push("a");
queue.push("b");
const first = queue[head];
head += 1;

Where each shows up

• Stack: valid parentheses, undo, depth-first search, expression evaluation
• Queue: breadth-first search, task scheduling, rate limiting
• Deque: sliding window maximum, where both ends are used
• Monotonic stack: next greater element and histogram problems

function isValid(text) {
  const pairs = { ")": "(", "]": "[", "}": "{" };
  const stack = [];
  for (const ch of text) {
    if (ch === "(" || ch === "[" || ch === "{") {
      stack.push(ch);
    } else if (ch in pairs) {
      if (stack.pop() !== pairs[ch]) return false;
    }
  }
  return stack.length === 0;
}