Below is a comprehensive JavaScript technical revision guide covering core concepts, control flow, data structures, functions, ES6+, async programming, common algorithms, and interview-style logic problems — with clear explanations and code snippets.

I’ve structured this like a mini handbook so you can revise section by section.

1. Variables and Data Types

`var`, `let`, `const`

var x = 10;     // function-scoped
let y = 20;     // block-scoped
const z = 30;   // block-scoped, cannot be reassigned

Scope Example

function testScope() {
    if (true) {
        var a = 1;
        let b = 2;
    }

    console.log(a); // 1
    // console.log(b); // ReferenceError
}

Primitive Types

string
number
boolean
undefined
null
bigint
symbol

let name = "John";
let age = 30;
let isActive = true;
let nothing = null;
let notDefined;

2. Operators

Arithmetic

let sum = 5 + 3;
let difference = 5 - 3;
let product = 5 * 3;
let division = 5 / 3;
let remainder = 5 % 3;
let power = 2 ** 3;

Comparison

5 == "5";   // true (loose)
5 === "5";  // false (strict)
5 !== 6;    // true

Logical

true && false;
true || false;
!true;

3. Conditional Statements

if / else

let score = 75;

if (score >= 90) {
    console.log("A");
} else if (score >= 70) {
    console.log("B");
} else {
    console.log("C");
}

Ternary Operator

let result = score >= 50 ? "Pass" : "Fail";

Switch

let day = 2;

switch(day) {
    case 1:
        console.log("Monday");
        break;
    case 2:
        console.log("Tuesday");
        break;
    default:
        console.log("Other");
}

4. Loops

for Loop

for (let i = 0; i < 5; i++) {
    console.log(i);
}

while Loop

let i = 0;

while (i < 5) {
    console.log(i);
    i++;
}

do…while

let i = 0;

do {
    console.log(i);
    i++;
} while (i < 5);

for…of (arrays)

let arr = [1, 2, 3];

for (let value of arr) {
    console.log(value);
}

for…in (objects)

let person = { name: "John", age: 30 };

for (let key in person) {
    console.log(key, person[key]);
}

5. Functions

Function Declaration

function greet(name) {
    return "Hello " + name;
}

Function Expression

const greet = function(name) {
    return "Hello " + name;
};

Arrow Functions

const greet = (name) => {
    return "Hello " + name;
};

// Short form
const greet = name => "Hello " + name;

Default Parameters

function greet(name = "Guest") {
    return `Hello ${name}`;
}

Rest Parameters

function sum(...numbers) {
    return numbers.reduce((total, num) => total + num, 0);
}

6. Arrays

Creating Arrays

let numbers = [1, 2, 3];
let empty = new Array();

Common Methods

push / pop

numbers.push(4);
numbers.pop();

shift / unshift

numbers.unshift(0);
numbers.shift();

map()

let doubled = numbers.map(n => n * 2);

filter()

let evens = numbers.filter(n => n % 2 === 0);

reduce()

let total = numbers.reduce((acc, curr) => acc + curr, 0);

find()

let found = numbers.find(n => n > 2);

some / every

numbers.some(n => n > 2);
numbers.every(n => n > 0);

7. Objects

Creating Objects

let person = {
    name: "John",
    age: 30,
    greet() {
        return `Hi, I'm ${this.name}`;
    }
};

Accessing Properties

person.name;
person["age"];

Destructuring

let { name, age } = person;

Spread Operator

let newPerson = { ...person, age: 31 };

8. Classes

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }

    greet() {
        return `Hello, I'm ${this.name}`;
    }
}

const p1 = new Person("John", 30);

Inheritance

class Student extends Person {
    constructor(name, age, course) {
        super(name, age);
        this.course = course;
    }
}

9. Closures

function outer() {
    let count = 0;

    return function inner() {
        count++;
        return count;
    };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

10. Higher-Order Functions

function operate(a, b, operation) {
    return operation(a, b);
}

function add(x, y) {
    return x + y;
}

operate(5, 3, add);

11. Promises

let promise = new Promise((resolve, reject) => {
    let success = true;

    if (success) {
        resolve("Success!");
    } else {
        reject("Error!");
    }
});

Using .then()

promise
    .then(result => console.log(result))
    .catch(error => console.error(error));

12. Async / Await

async function fetchData() {
    try {
        let response = await fetch("https://api.example.com/data");
        let data = await response.json();
        console.log(data);
    } catch (error) {
        console.error(error);
    }
}

13. Error Handling

try {
    throw new Error("Something went wrong");
} catch (error) {
    console.error(error.message);
} finally {
    console.log("Cleanup");
}

14. Common Algorithm Questions

Reverse a String

function reverseString(str) {
    return str.split("").reverse().join("");
}

Check Palindrome

function isPalindrome(str) {
    let reversed = str.split("").reverse().join("");
    return str === reversed;
}

Find Maximum in Array

function findMax(arr) {
    return Math.max(...arr);
}

Remove Duplicates

function removeDuplicates(arr) {
    return [...new Set(arr)];
}

FizzBuzz

for (let i = 1; i <= 100; i++) {
    if (i % 3 === 0 && i % 5 === 0) {
        console.log("FizzBuzz");
    } else if (i % 3 === 0) {
        console.log("Fizz");
    } else if (i % 5 === 0) {
        console.log("Buzz");
    } else {
        console.log(i);
    }
}

Factorial

function factorial(n) {
    if (n === 0) return 1;
    return n * factorial(n - 1);
}

Fibonacci

function fibonacci(n) {
    if (n <= 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

15. Sorting

let numbers = [5, 3, 8, 1];

numbers.sort((a, b) => a - b);  // ascending
numbers.sort((a, b) => b - a);  // descending

16. Deep vs Shallow Copy

let obj = { a: 1, b: { c: 2 } };

let shallowCopy = { ...obj };
let deepCopy = JSON.parse(JSON.stringify(obj));

17. Event Loop Concept (Important for Interviews)

JavaScript is single-threaded.

console.log("Start");

setTimeout(() => {
    console.log("Timeout");
}, 0);

console.log("End");

Output:

Start
End
Timeout

18. Debounce Function (Common Interview Question)

function debounce(func, delay) {
    let timeoutId;

    return function(...args) {
        clearTimeout(timeoutId);
        timeoutId = setTimeout(() => {
            func.apply(this, args);
        }, delay);
    };
}

19. Throttle Function

function throttle(func, limit) {
    let lastFunc;
    let lastRan;

    return function(...args) {
        if (!lastRan) {
            func.apply(this, args);
            lastRan = Date.now();
        } else {
            clearTimeout(lastFunc);
            lastFunc = setTimeout(() => {
                if ((Date.now() - lastRan) >= limit) {
                    func.apply(this, args);
                    lastRan = Date.now();
                }
            }, limit - (Date.now() - lastRan));
        }
    };
}

20. Common Array Logic Problems

Count Occurrences

function countOccurrences(arr) {
    return arr.reduce((acc, curr) => {
        acc[curr] = (acc[curr] || 0) + 1;
        return acc;
    }, {});
}

Flatten Array

function flatten(arr) {
    return arr.flat(Infinity);
}

Recursive version:

function flatten(arr) {
    return arr.reduce((acc, curr) =>
        Array.isArray(curr)
            ? acc.concat(flatten(curr))
            : acc.concat(curr), []
    );
}

21. Two Sum Problem

function twoSum(nums, target) {
    let map = {};

    for (let i = 0; i < nums.length; i++) {
        let complement = target - nums[i];

        if (map[complement] !== undefined) {
            return [map[complement], i];
        }

        map[nums[i]] = i;
    }
}

22. Memoization

function memoize(fn) {
    let cache = {};

    return function(...args) {
        let key = JSON.stringify(args);

        if (!cache[key]) {
            cache[key] = fn.apply(this, args);
        }

        return cache[key];
    };
}

23. Practical Interview Scenario Example

Filter + Sort + Transform

const users = [
    { name: "John", age: 25 },
    { name: "Alice", age: 30 },
    { name: "Bob", age: 20 }
];

const result = users
    .filter(user => user.age > 21)
    .sort((a, b) => a.age - b.age)
    .map(user => user.name);

console.log(result);

Final Advice for Your Technical Test

Practice writing code without autocomplete.
Be comfortable with:
- Loops
- Array methods
- Objects
- Async/await
- Common algorithms
Speak while coding (if live interview).
Explain time complexity when appropriate.
Handle edge cases (null, empty array, invalid input).

JavaScript