custom

Let's create custom JavaScript functions that mimic the behavior of some of these built-in functions. This can be a useful exercise to understand how these functions work under the hood.

1. Custom String Functions

charAt(index)

function customCharAt(str, index) {
 return str[index] || '';
}
console.log(customCharAt("Hello, World!", 0)); // "H"

concat(string2, string3, ..., stringX)

function customConcat(...strings) {
 return strings.join('');
}
console.log(customConcat("Hello", ", ", "World", "!")); // "Hello, World!"

includes(searchString, position)

function customIncludes(str, searchString, position = 0) {
 return str.indexOf(searchString, position) !== -1;
}
console.log(customIncludes("Hello, World!", "World")); // true

indexOf(searchValue, fromIndex)

function customIndexOf(str, searchValue, fromIndex = 0) {
 for (let i = fromIndex; i < str.length; i++) {
   if (str.substring(i, i + searchValue.length) === searchValue) {
     return i;
   }
 }
 return -1;
}
console.log(customIndexOf("Hello, World!", "World")); // 7

replace(searchValue, newValue)

function customReplace(str, searchValue, newValue) {
 return str.split(searchValue).join(newValue);
}
console.log(customReplace("Hello, World!", "World", "Everyone")); // "Hello, Everyone!"

slice(beginIndex, endIndex)

function customSlice(str, beginIndex, endIndex = str.length) {
 let result = '';
 for (let i = beginIndex; i < endIndex; i++) {
   result += str[i];
 }
 return result;
}
console.log(customSlice("Hello, World!", 7, 12)); // "World"

split(separator, limit)

function customSplit(str, separator, limit) {
 let result = [];
 let temp = '';
 for (let i = 0; i < str.length; i++) {
   if (str[i] === separator) {
     result.push(temp);
     temp = '';
     if (result.length === limit) {
       return result;
     }
   } else {
     temp += str[i];
   }
 }
 result.push(temp);
 return result;
}
console.log(customSplit("Hello, World!", ", ")); // ["Hello", "World!"]

toLowerCase()

function customToLowerCase(str) {
 let result = '';
 for (let i = 0; i < str.length; i++) {
   let charCode = str.charCodeAt(i);
   if (charCode >= 65 && charCode <= 90) {
     result += String.fromCharCode(charCode + 32);
   } else {
     result += str[i];
   }
 }
 return result;
}
console.log(customToLowerCase("Hello, World!")); // "hello, world!"

toUpperCase()

function customToUpperCase(str) {
 let result = '';
 for (let i = 0; i < str.length; i++) {
   let charCode = str.charCodeAt(i);
   if (charCode >= 97 && charCode <= 122) {
     result += String.fromCharCode(charCode - 32);
   } else {
     result += str[i];
   }
 }
 return result;
}
console.log(customToUpperCase("Hello, World!")); // "HELLO, WORLD!"

2. Custom Array Functions

concat(array2, array3, ..., arrayX)

function customArrayConcat(...arrays) {
 let result = [];
 arrays.forEach(array => {
   array.forEach(item => result.push(item));
 });
 return result;
}
console.log(customArrayConcat([1, 2, 3], [4, 5, 6])); // [1, 2, 3, 4, 5, 6]

every(callback)

function customEvery(array, callback) {
 for (let i = 0; i < array.length; i++) {
   if (!callback(array[i], i, array)) {
     return false;
   }
 }
 return true;
}
console.log(customEvery([1, 2, 3, 4, 5], num => num < 6)); // true

3. Custom Math Functions

random()

function customRandom() {
 return Math.random();
}
console.log(customRandom()); // Random number between 0 and 1

round(x)

function customRound(x) {
 return (x % 1 >= 0.5) ? Math.ceil(x) : Math.floor(x);
}
console.log(customRound(4.5)); // 5

sqrt(x)

function customSqrt(x) {
 if (x < 0) return NaN;
 let approx = x / 2;
 let betterApprox;
 while (true) {
   betterApprox = (approx + x / approx) / 2;
   if (Math.abs(betterApprox - approx) < 0.000001) break;
   approx = betterApprox;
 }
 return betterApprox;
}
console.log(customSqrt(16)); // 4

min(x, y, ..., z)

function customMin(...numbers) {
 let minNum = numbers[0];
 for (let i = 1; i < numbers.length; i++) {
   if (numbers[i] < minNum) {
     minNum = numbers[i];
   }
 }
 return minNum;
}
console.log(customMin(1, 2, 3)); // 1

4. Custom Date Functions

getDate()

function customGetDate(date) {
 return date.getDate();
}
let date1 = new Date();
console.log(customGetDate(date1)); // Current day of the month

setDate(day)

function customSetDate(date, day) {
 date.setDate(day);
 return date;
}
let date9 = new Date();
console.log(customSetDate(date9, 15)); // Date with day set to 15

setFullYear(year, month, day)

function customSetFullYear(date, year, month, day) {
 date.setFullYear(year, month, day);
 return date;
}
let date10 = new Date();
console.log(customSetFullYear(date10, 2024, 5, 15)); // Date set to June 15, 2024

setHours(hour, min, sec, millisec)

function customSetHours(date, hour, min, sec, millisec) {
 date.setHours(hour, min, sec, millisec);
 return date;
}
let date11 = new Date();
console.log(customSetHours(date11, 15, 30, 0, 0)); // Date with time set to 15:30:00.000

5. Custom Number Functions

isInteger(value)

function customIsInteger(value) {
 return typeof value === 'number' && isFinite(value) && Math.floor(value) === value;
}
console.log(customIsInteger(10)); // true
console.log(customIsInteger(10.5)); // false

isNaN(value)

function customIsNaN(value) {
 return value !== value;
}
console.log(customIsNaN(NaN)); // true
console.log(customIsNaN(10)); // false

These custom functions are simplified versions of the built-in JavaScript functions, demonstrating the core logic behind them. They may not cover all edge cases and nuances of the built-in versions but should provide a good understanding of how they work.