Intern and Fresher Developer Interview Questions and Answers

var is function-scoped and can be redeclared; let and const are block-scoped, and const cannot be reassigned after it is set.

== compares values after type coercion, while === compares both value and type with no conversion, so it is the safer default.

Hoisting is JavaScript moving declarations to the top of their scope during compilation, so functions and var variables can be referenced before the line where they are written.

undefined means a variable has been declared but no value was assigned; null is an explicit value meaning 'no value' that you assign on purpose.

map transforms every item into a new array of the same length, filter keeps only items that pass a test, and reduce folds the whole array into a single value.

The virtual DOM is a lightweight JavaScript copy of the UI that React compares with the previous version after each update, so it only changes the parts of the real DOM that actually differ.

Props are read-only inputs passed from a parent component, while state is data a component owns and can change over time, which triggers a re-render when updated.

Semantic elements describe their meaning in the name, such as header, nav, main, article and footer, so browsers, assistive technology and search engines can understand the page structure.

Every element is a box made of content, padding, border and margin. The default content-box size counts only the content; border-box includes padding and border in the width you set.

display: none removes the element from the layout so it takes no space; visibility: hidden hides it but leaves the empty space, and the element can still be read in the accessibility tree in some cases.

Block elements start on a new line and take the full available width; inline elements sit in the text flow and ignore width and height; inline-block sits in the flow but accepts width, height, padding and margin on all sides.

TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. It catches type errors in the editor and at build time, and it documents the shape of data for the next person who reads the code.

Both describe object shapes. interface can be merged and is preferred for public object contracts; type can express unions, intersections and primitives, and cannot be reopened later.

INNER JOIN returns only rows that have a match in both tables, while LEFT JOIN returns every row from the left table and fills the right table's columns with NULL when there is no match.

WHERE filters individual rows before grouping, while HAVING filters groups after GROUP BY, so only HAVING can use aggregate functions like COUNT or SUM.

DELETE removes selected rows and can be rolled back, TRUNCATE quickly removes all rows and keeps the table structure, and DROP removes the entire table including its structure.

A primary key uniquely identifies each row in its own table and cannot be NULL, while a foreign key is a column that references the primary key of another table to link the two.

A value type variable holds its data directly and is copied on assignment, while a reference type variable holds a reference to an object, so assignment copies the reference and both variables see the same object.

A string is immutable, so every change creates a new string object, while StringBuilder is a mutable buffer that modifies its contents in place, making it much faster for repeated concatenation.

The JVM runs Java bytecode, the JRE is the JVM plus the standard libraries needed to run Java programs, and the JDK is the JRE plus the development tools such as the compiler (javac).

== compares primitives by value and objects by reference (whether they are the exact same object), while equals() compares the logical content of two objects when the class overrides it.

String is immutable, StringBuilder is a mutable and fast but not thread-safe buffer, and StringBuffer is a mutable buffer with synchronized methods that is thread-safe but slower.

final is a keyword that prevents reassigning a variable, overriding a method or extending a class; finally is a block that always runs after try/catch for cleanup; finalize() was a method called by the garbage collector and is now deprecated.

A list is mutable and can be changed after creation, while a tuple is immutable; tuples are slightly faster, use less memory and can be used as dictionary keys when their items are hashable.

== checks whether two objects have equal values, while is checks whether they are the exact same object in memory.

Node.js is a JavaScript runtime built on Chrome's V8 engine that runs your JavaScript on a single main thread and uses non-blocking I/O, so one thread can handle many concurrent connections.

package.json lists your project's dependencies with allowed version ranges, while package-lock.json records the exact versions that were installed so every machine gets an identical dependency tree.