Top 20 FrontEnd Interview Questions 2026 | Front End Developer Interview Questions And Answers 2026

Main ideas / lessons (Top 20 Front-End Interview Questions, summarized)

The video presents a set of common front-end interview questions (primarily JavaScript, React, Redux, HTML/CSS, and general async/API handling) and gives brief solution approaches for each.

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Methodologies & instructions

1) Remove duplicate values from an array (JavaScript)

• Create a new array (e.g., uniqueAr).
• Use the spread operator to copy array elements into a Set.
• Use Set to keep only unique values.
• Spread the Set back into an array to produce the unique result.

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2) Explain == vs === in JavaScript

• == (double equals):
  • Checks only value (loose equality).
• === (triple equals):
  • Checks value and type (strict equality).

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3) Implement a debounce function (JavaScript)

• Debounce delays function execution until after events stop for a given time.
• Purpose:
  • Limit how often a function runs for rapidly triggered events (scroll, resize, typing).
  • Reduce performance issues from excessive calls.
• Typical structure shown:
  • Create debounce(functionToDebounce, delay)
  • Maintain a timeout variable.
  • On each call:
    • Store arguments/context
    • Clear previous timeout
    • Set a new setTimeout to invoke the function after delay
    • Use apply (with context + saved arguments)

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4) Check whether a string is a palindrome (JavaScript)

• Take input as string.
• Create a reversed version:
  • Split into characters, reverse, and join.
• Compare:
  • If original string equals reversed string → palindrome
  • Otherwise → not a palindrome

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5) Deep clone an object (JavaScript)

• Use JSON serialization:
  • JSON.parse(JSON.stringify(obj))
• Limitation mentioned:
  • Does not support functions and does not properly handle undefined.

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6) Difference between null and undefined (JavaScript)

• null:
  • Explicitly assigned to represent absence of value.
• undefined:
  • Variable is declared but not assigned a value (value is missing/uninitialized).

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7) Difference between event bubbling and event capturing (JavaScript)

• Event propagation phases:
  1. Capturing phase: event travels from the topmost ancestor (document/window) down toward the target.
  2. Target phase: event reaches the actual target element.
  3. Bubbling phase: event travels upward from target to ancestors.
• Bubbling:
  • From target → outward to parent/ancestors.
• Capturing:
  • From ancestor → downward to the target.

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8) Difference between call, apply, and bind (JavaScript)

• call:
  • Invokes immediately
  • Sets this
  • Arguments passed individually
• apply:
  • Similar to call
  • Invokes immediately
  • Sets this
  • Arguments passed as an array
• bind:
  • Returns a new function with this set
  • Does not invoke immediately
  • Useful when you want to run later

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9) Optimize React component performance

• Use:
  • React.memo for functional components
  • useCallback to memoize functions
  • useMemo to memoize derived calculations
• Avoid:
  • Unnecessary state updates

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10) What are hooks in React and why use them?

• Hooks enable functional components to:
  • Manage state and lifecycle-like behavior
  • Without class components
• Examples mentioned:
  • useState, useEffect, useRef

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11) Difference between useEffect dependency usage and “mount only”

• useEffect(callback, []) (empty dependency array):
  • Runs on mount only
• useEffect(callback, [dependency]):
  • Runs again when the dependency changes
• The effect is described as side-effect behavior.

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12) Prevent re-rendering of a child component (React)

• Use React.memo on the child component to avoid re-rendering when props haven’t changed.
• Use useCallback for passing stable function references as props.
• Use useMemo for stable derived values to reduce prop changes.

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13) Explain Virtual DOM and how React uses it

• React creates a Virtual DOM (lightweight representation of the real DOM).
• On state change:
  • React compares Virtual DOM vs actual DOM
  • Updates only the changed parts in the actual DOM

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14) What is Redux and why used?

• Redux is a state management library
• Stores global state in a single store
• Benefits mentioned:
  • Better state predictability
• Typically used alongside React (as implied by the video).

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15) Center a <div> horizontally and vertically (CSS)

• Use Flexbox:
  • display: flex
  • justify-content: center (horizontal centering)
  • align-items: center (vertical centering)

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16) Create a responsive grid layout (CSS Grid)

• Use CSS Grid with:
  • grid-template-columns
  • repeat(...)
  • auto-fill and minmax(...)
  • A gap (example: 20px)
• Goal:
  • Responsive columns that adjust based on available space.

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17) Gradually change background color on hover (CSS)

• Use the CSS transition property.
• Example logic from the subtitles:
  • Set background to initial color (e.g., blue)
  • Apply transition (e.g., duration like 0.3s and easing like ease-in-out)
  • On :hover, change background to another color (e.g., red)

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18) Create a sticky header (CSS)

• Use position: sticky
• Example behavior described:
  • top: 0
  • set background (e.g., white)
  • set z-index to keep it on top

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19) Difference between relative, absolute, fixed, and sticky positioning (CSS)

• relative:
  • Moves relative to its normal position.
• absolute:
  • Positioned relative to the nearest positioned ancestor.
• fixed:
  • Stays in place even while scrolling.
• sticky:
  • “Sticks” to a position once you scroll past it.

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20) Handle API calls in JavaScript (async/await + try/catch)

• Use an async function to fetch data.
• Wrap calls in try / catch for error handling.
• Use await on the fetch request (and response parsing).
• On success:
  • log/return the received data
• On failure:
  • log/handle the error
• Approach described as using fetch with async/await and try/catch for cleaner code and error handling.

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Speakers / sources featured

• Simply Learn (channel/brand mentioned repeatedly)
• IBM (referenced as a collaboration for the promoted full-stack course)