List of Mostly Used Inbuilt JS Functions

Here is the full list of mostly used JavaScript inbuilt functions along with their definitions and usage examples, categorized for easy reading.

✅ ARRAYS

const arr = [1, 2, 3];

// 1. push() – Adds element to the end
arr.push(4); // [1,2,3,4]

// 2. pop() – Removes last element
arr.pop(); // [1,2,3]

// 3. shift() – Removes first element
arr.shift(); // [2,3]

// 4. unshift() – Adds element at the start
arr.unshift(1); // [1,2,3]

// 5. concat() – Merges arrays
arr.concat([4, 5]); // [1,2,3,4,5]

// 6. slice() – Extracts a section (non-destructive)
arr.slice(0, 2); // [1,2]

// 7. splice() – Adds/removes items (destructive)
arr.splice(1, 1); // removes 1 element at index 1

// 8. indexOf() – Finds index of an element
arr.indexOf(3); // returns index or -1

// 9. filter() – Filters based on a condition
arr.filter(x => x > 1);

// 10. map() – Transforms each element
arr.map(x => x * 2);

// 11. Array.from() – Converts iterable/string to array
Array.from('abc'); // ['a','b','c']

// 12. toString() – Converts array to comma string
arr.toString(); // "1,3"

// 13. includes() – Checks for existence
arr.includes(3); // true

// 14. sort() – Sorts array
arr.sort(); // in-place sort

// 15. reverse() – Reverses array
arr.reverse();

// 16. join() – Joins into a string
arr.join('-'); // "1-3"

// 17. find() – Finds first matching element
arr.find(x => x > 1);

// 18. forEach() – Loops through elements
arr.forEach(x => console.log(x));

// 19. every() – Checks if all elements meet condition
arr.every(x => x > 0);

// 20. some() – Checks if any meet condition
arr.some(x => x === 3);

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✅ STRINGS

let str = "hello";

// 1. charAt() – Gets character by index
str.charAt(1); // 'e'

// 2. toUpperCase() – Uppercase
str.toUpperCase();

// 3. toLowerCase() – Lowercase
str.toLowerCase();

// 4. indexOf() – First occurrence index
str.indexOf('l');

// 5. lastIndexOf() – Last occurrence index
str.lastIndexOf('l');

// 6. substring() – Slice string between indexes
str.substring(1, 4);

// 7. replace() – Replace part of string
str.replace('e', 'a');

// 8. split() – Split into array
str.split(''); // ['h','e','l','l','o']

// 9. trim() – Removes whitespace
"  hello  ".trim(); // "hello"

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✅ OBJECTS

const obj = { a: 1, b: 2 };

// 1. Object.keys() – Array of keys
Object.keys(obj);

// 2. Object.values() – Array of values
Object.values(obj);

// 3. Object.entries() – Array of [key, value]
Object.entries(obj);

// 4. Object.assign() – Merge objects
Object.assign({}, obj);

// 5. Object.freeze() – Lock object (no change)
Object.freeze(obj);

// 6. Object.seal() – Prevent add/delete
Object.seal(obj);

// 7. hasOwnProperty() – Check own property
obj.hasOwnProperty('a');

// 8. Object.create() – Create with prototype
const newObj = Object.create(obj);

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✅ DATE

const date = new Date();

// 1. new Date() – Current date/time
// 2. getDate() – Day of the month
date.getDate();

// 3. getMonth() – Month (0-11)
date.getMonth();

// 4. getFullYear() – Year
date.getFullYear();

// 5. getHours(), getMinutes(), getSeconds()
date.getHours();
date.getMinutes();
date.getSeconds();

// 6. toISOString() – ISO string
date.toISOString();

// 7. toLocaleDateString() – Localized date
date.toLocaleDateString();

// 8. getTime() – ms since 1970
date.getTime();

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✅ MATH

// 1. Math.abs() – Absolute value
Math.abs(-5);

// 2. Math.round() – Round to nearest
Math.round(4.5);

// 3. Math.floor() – Round down
Math.floor(4.9);

// 4. Math.ceil() – Round up
Math.ceil(4.2);

// 5. Math.max(), Math.min()
Math.max(1, 2, 3);
Math.min(1, 2, 3);

// 6. Math.pow(), Math.sqrt()
Math.pow(2, 3); // 8
Math.sqrt(16); // 4

// 7. Math.random() – 0 to 1
Math.random();

// 8. Math.PI
Math.PI;

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✅ SET

const s = new Set();

// 1. add(), delete(), has()
s.add(1);
s.delete(1);
s.has(1);

// 2. size – Count
s.size;

// 3. clear() – Remove all
s.clear();

// 4. forEach() – Iterate
s.forEach(val => console.log(val));

// 5. values(), entries()
[...s.values()];
[...s.entries()];

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✅ MAP

const m = new Map();

// 1. set(), get(), has(), delete()
m.set('a', 1);
m.get('a');
m.has('a');
m.delete('a');

// 2. size – Count
m.size;

// 3. clear()
m.clear();

// 4. forEach(), keys(), values(), entries()
m.forEach((v, k) => console.log(k, v));
[...m.keys()];
[...m.values()];
[...m.entries()];

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✅ LOOPS

const obj = { a: 1, b: 2 };

// 1. for...in – Loop over object keys
for (let key in obj) {
  console.log(key);
}

// 2. for...of – Loop over iterable values
for (let val of [1, 2, 3]) {
  console.log(val);
}

// 3. forEach – Loop over array items
[1, 2, 3].forEach(x => console.log(x));

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Comments

[Tracy Gilmore]

My apologies but I am not sure what this post is intended to offer. Readers might be better off reading an authoritative source such as MDN