javascript-test.md

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1. What are the different data types in JavaScript?

Answer:

• Primitive types: String, Number, Boolean, Undefined, Null, Symbol, BigInt.
• Non-primitive type: Object (includes arrays, functions, etc.).

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2. Explain the difference between var, let, and const.

Answer:

• var: Function-scoped, can be re-declared, and hoisted.
• let: Block-scoped, cannot be re-declared, and allows reassignment.
• const: Block-scoped, cannot be re-declared or reassigned.

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3. What is the difference between == and ===?

Answer:

• ==: Checks for equality after type coercion (loose equality).
• ===: Checks for equality without type coercion (strict equality).

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4. What is a closure in JavaScript?

Answer: A closure is a function that retains access to its outer scope, even after the outer function has finished executing.
Example:

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}
const increment = outer();
console.log(increment()); // 1
console.log(increment()); // 2

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5. What is the difference between undefined and null?

Answer:

• undefined: A variable that has been declared but not assigned a value.
• null: An explicitly assigned value representing no value or absence of an object.

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6. What is event bubbling and event capturing?

Answer:

• Event Bubbling: Events propagate from the innermost element (target) to the outermost (parent).
• Event Capturing: Events propagate from the outermost element (parent) to the innermost (target).

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7. What is the output of the following code?

console.log(0.1 + 0.2 === 0.3);

Answer: false
Explanation: Due to floating-point precision errors, 0.1 + 0.2 results in 0.30000000000000004.

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8. Explain call(), apply(), and bind().

Answer:

• call(): Calls a function with a given this value and arguments passed individually.
  Example: func.call(thisArg, arg1, arg2).
• apply(): Similar to call() but takes arguments as an array.
  Example: func.apply(thisArg, [arg1, arg2]).
• bind(): Returns a new function with a given this value and arguments pre-set.
  Example: const boundFunc = func.bind(thisArg, arg1).

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9. What are arrow functions, and how are they different from regular functions?

Answer: Arrow functions are a concise way to write functions.
Key differences:

1. Syntax: const func = () => { ... }.
2. this binding: Arrow functions do not bind their own this; they inherit it from the surrounding context.
3. Cannot be used as constructors.

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10. What will the following code output and why?

let a = [];
let b = [];
console.log(a == b, a === b);

Answer: false false
Explanation: Arrays are objects, and object comparison checks for reference equality. Since a and b are different objects, they are not equal.

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These questions cover essential JavaScript concepts and should give you a good sense of the rookie's understanding.