Added 10 advanced JavaScript interview question examples with explanations

interview-questions/01_hoisting.js

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+// ❓ Question: What is hoisting in JavaScript?
+
+console.log(a); // ?
+var a = 10;
+
+hoistedFunction(); // ?
+
+function hoistedFunction() {
+  console.log("I am hoisted!");
+}
+
+/*
+🧩 Output:
+undefined
+I am hoisted!
+
+🧠 Explanation:
+- Variable declarations using `var` are hoisted to the top but not their assignments (so `a` is undefined).
+- Function declarations are fully hoisted — both definition and declaration.
+*/

interview-questions/02_closures.js

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+// ❓ Question: What is a closure? Explain with example.
+
+function outer() {
+  let counter = 0;
+  return function inner() {
+    counter++;
+    console.log(counter);
+  };
+}
+
+const fn = outer();
+fn(); // 1
+fn(); // 2
+fn(); // 3
+
+/*
+🧩 Output:
+1
+2
+3
+
+🧠 Explanation:
+- A closure is created when a function retains access to its parent scope even after that parent has finished executing.
+- Here, `inner` remembers `counter` from `outer`.
+*/

interview-questions/03_this_keyword.js

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+// ❓ Question: What does `this` refer to in different contexts?
+
+const obj = {
+  name: "Alice",
+  show: function () {
+    console.log(this.name);
+  },
+};
+
+obj.show(); // ?
+const fn = obj.show;
+fn(); // ?
+
+/*
+🧩 Output:
+Alice
+undefined
+
+🧠 Explanation:
+- Inside `obj.show()`, `this` refers to `obj`.
+- When detached (`fn()`), `this` refers to the global object (or undefined in strict mode).
+*/

interview-questions/04_event_loop.js

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+// ❓ Question: What is the order of output?
+
+console.log("Start");
+
+setTimeout(() => console.log("Timeout"), 0);
+
+Promise.resolve().then(() => console.log("Promise"));
+
+console.log("End");
+
+/*
+🧩 Output:
+Start
+End
+Promise
+Timeout
+
+🧠 Explanation:
+- JS executes synchronous code first.
+- Then, microtasks (Promises).
+- Finally, macrotasks (setTimeout).
+*/

interview-questions/05_async_await.js

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+// ❓ Question: Explain async/await behavior.
+
+async function test() {
+  console.log("1");
+  await Promise.resolve();
+  console.log("2");
+}
+test();
+console.log("3");
+
+/*
+🧩 Output:
+1
+3
+2
+
+🧠 Explanation:
+- `await` pauses only the async function, not the whole script.
+- The rest of the code (`console.log("3")`) runs first.
+*/

interview-questions/06_object_comparison.js

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+// ❓ Question: Why does object comparison return false?
+
+const a = { x: 1 };
+const b = { x: 1 };
+
+console.log(a == b);  // ?
+console.log(a === b); // ?
+
+/*
+🧩 Output:
+false
+false
+
+🧠 Explanation:
+Objects are compared by reference, not by value.
+Each `{ x: 1 }` creates a new object at a different memory address.
+*/

interview-questions/07_type_coercion.js

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+// ❓ Question: What will be the output?
+
+console.log(1 + "2" + "2");
+console.log(1 + +"2" + "2");
+console.log(1 + -"1" + "2");
+console.log(+"1" + "1" + "2");
+console.log("A" - "B" + "2");
+console.log("A" - "B" + 2);
+
+/*
+🧩 Output:
+122
+32
+02
+112
+NaN2
+NaN
+
+🧠 Explanation:
+- JS automatically converts types in expressions.
+- `"A" - "B"` gives NaN because strings can't be subtracted.
+*/

interview-questions/08_function_scope.js

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+// ❓ Question: What happens to variable scope?
+
+function test() {
+  var a = 10;
+  if (true) {
+    let b = 20;
+    var c = 30;
+  }
+  console.log(a); // ?
+  // console.log(b); // ?
+  console.log(c); // ?
+}
+test();
+
+/*
+🧩 Output:
+10
+30
+Error if b is logged
+
+🧠 Explanation:
+- `var` is function-scoped.
+- `let` and `const` are block-scoped.
+*/

interview-questions/09_promise_chaining.js

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+// ❓ Question: Predict the output of chained Promises
+
+Promise.resolve()
+  .then(() => {
+    console.log("1");
+    return "2";
+  })
+  .then((data) => {
+    console.log(data);
+    throw new Error("Something went wrong");
+  })
+  .catch(() => console.log("3"))
+  .then(() => console.log("4"));
+
+/*
+🧩 Output:
+1
+2
+3
+4
+
+🧠 Explanation:
+- `.catch()` handles the error and then continues with the next `.then()`.
+*/

interview-questions/10_destructuring.js

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+// ❓ Question: What is destructuring and how does it work?
+
+const user = { name: "Bob", age: 25, city: "Delhi" };
+
+const { name, age } = user;
+console.log(name, age);
+
+const arr = [10, 20, 30];
+const [x, , y] = arr;
+console.log(x, y);
+
+/*
+🧩 Output:
+Bob 25
+10 30
+
+🧠 Explanation:
+Destructuring allows unpacking properties or array items into variables easily.
+*/