JavaScript Interview Questions for Freshers

const locks the binding: you can't point the name at a new value. It does not freeze the value itself, so const user = {} still lets you write user.name = 'Ana'. Uppercase names are only a convention for fixed settings, and redeclaring any let or const name in the same scope is a SyntaxError. Use Object.freeze when you need the object itself locked.

Arrays are a special kind of object in JavaScript, so typeof reports "object". There is no "array" result. To check for an array, use Array.isArray(value), which returns true only for real arrays, including ones created in another frame or window. This is one of the first type questions in most fresher rounds, often asked together with typeof null.

When either side of + is a string, JavaScript joins them as text, so "5" + 3 is the string "53". The minus operator only works on numbers, so "5" - 3 converts the string to 5 first and gives 2. This difference is a classic source of bugs with form inputs, which are always strings. Convert explicitly with Number(value) before doing arithmetic.

True. Loose equality treats null and undefined as equal to each other and to nothing else, so null == 0 and undefined == false are both false. That is why value == null is a common, deliberate shortcut for "null or undefined". With strict equality, null === undefined is false because they are different types.

Array.isArray is built for exactly this check and works even for arrays from another iframe. typeof never returns "array". Checking length also matches strings and objects that happen to have a length property. instanceof Object is true for almost everything that isn't a primitive, including plain objects, functions and dates, so it can't tell an array apart.

JavaScript has seven primitive types: string, number, bigint, boolean, undefined, symbol and null. Primitives are immutable and compared by value. Arrays are not a separate type: they are objects, and objects are compared by reference. Knowing the list matters for questions about copying, equality and typeof, and interviewers often add array as a trap option.

The inner let x lives only inside the curly braces, so it is a separate variable that shadows the outer one there. After the block ends, the outer x is still 1. Declaring the same name in a nested block is allowed; only redeclaring it in the same scope throws. With var instead of let both lines would share one function-scoped variable and the output would be 2.

Objects are compared by reference, not by content. Each literal creates a new object in memory, so the two sides are different objects and === returns false, even though they look identical. Two variables are only equal if they point to the same object. To compare contents, compare the fields you care about or use a deep-equality helper.

push and pop work at the end of the array; unshift and shift work at the start. All four change the original array. push and unshift return the new length, while pop and shift return the removed item. Working at the start is slower on big arrays because every other item has to move one place.

By default, events bubble: they fire on the element that was clicked, then on each ancestor up to the document. So the button handler runs before the div handler. Capturing, the opposite direction, only happens if you pass { capture: true }. You can stop the climb with event.stopPropagation(). Bubbling is what makes event delegation possible.