Advanced-JavaScript-With-Projects/arrayMethodCallBackFunction.html at main · marleyDip/Advanced-JavaScript-With-Projects · GitHub

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Array Method & CallBack Function</title>
  </head>
  <body></body>
  <script>
    /* forEach() method => To iterate over each item in an array and perform a side effect (like logging, updating, etc.)

    => Return value: undefined — it doesn't return a new array.

    => Used for: side effects (e.g., printing, modifying variables, calling functions) */

    // A function without name is called anonymous function.

    // forEach (parameter) argument call anonymous function thats why is called callback function

    // l = [], value as argument for anonymous function.....it takes or pass single value & execute.

    let l = [10, 20, 30, 40, 50];

    l.forEach((v, i) => {
      console.log(v * 3, i);

      // here call anonymous function (10), (20), (30), (40), (50)

      //1st parameter value, 2nd parameter index number
    });

    /* map() method => To transform each item in the array and return a new array

    => Return value: a new array of the same length

    => Used for: data transformation */

    let s = [10, 20, 30, 40, 50];

    let finalAns = s.map((v, i) => v + 5);

    console.log(finalAns); // [ 15, 25, 35, 45, 55 ]
    console.log(s); // [10, 20, 30, 40, 50]

    let seeResults = (v, i) => {
      return v + i; // 10 + 0 = 10, 20 + 1 = 21, ...
    };

    let finalAns1 = s.map(seeResults);
    console.log(finalAns1); // [ 10, 21, 32, 43, 54 ]
    console.log(s); // [10, 20, 30, 40, 50]

    /* filter() =>  To create a new array containing only the elements that match a condition

    => Return value: A new array (may be empty or smaller)

    => Used for: Selecting specific items based on criteria
    */

    let d = [99, 77, 66, 55, 44, 42, 55, 88];

    let filterData = d.filter((v, i) => v % 2 == 0);

    console.log(filterData); // [ 66, 44, 42, 88 ]
    console.log(d); // [99, 77, 66, 55, 44, 42, 55, 88]

    let filterData1 = d.filter((num) => {
      if (num % 2 === 1) {
        // return num;
        return true;
      }
    });

    console.log(filterData1); // [ 99, 77, 55, 55 ]
    console.log(d); // [99, 77, 66, 55, 44, 42, 55, 88]

    // The filter() method creates a new array with array elements that pass a test.

    let numbers = [45, 4, 9, 16, 25];

    console.log(numbers.filter((v) => v > 4));

    let over9 = numbers.filter((v) => v > 9);
    console.log(over9);

    const over18 = numbers.filter(myFunction);
    console.log(over18);

    function myFunction(value) {
      return value > 18;
    }

    /* reduce() => The reduce() method runs a function on each array element to produce (reduce it to) a single value.

    => The reduce() method works from left-to-right in the array.

    => The reduce() method can accept an initial value

    => To reduce an array to a single value (number, object, array, string, etc.)

    => Return value: Whatever value you return from the reducer — can be anything

    => Used for: Summing, multiplying, merging, flattening, counting, etc.
    */

    /*
    array.reduce((accumulator, currentValue) => {

        accumulator => (the initial value / previously returned value)

        currentValue => (array present value)
    return updated accumulator

    }, initialValue);
    */

    const num = [45, 4, 9, 16, 25];

    // for...of

    let num1 = 0;
    //num1 = 0;  // ← This becomes global (bad practice)

    for (let nums of num) {
      // Loops over each number in the array (nums takes values like 45, 4, 9, ...)
      num1 += nums;
    }
    console.log(num1); // 99

    let sum = num.reduce(numOfTotal, 100);
    console.log("the total of num: ", sum); // 199

    function numOfTotal(total, value) {
      return total + value;
    }

    /*
      Cannot access 'numTotal' before initialization
        let sum1 = num.reduce(numTotal);
    */

    let numTotal = (total, value) => total - value; // 200 - 45 = 155; 155 - 4 = 151; 151 - 9 = 142; 142 - 16 = 126; 126 - 25 = 101

    let sum1 = num.reduce(numTotal, 200); // here 200 is initial value
    console.log("the total of num: ", sum1); // 101

    console.log(num.reduce((total, value) => total + value, 100));

    let final = num.reduce((total, value) => {
      return total + value;
    }, 100);
    console.log(final);

    console.log(num.reduce((total, value) => total + value)); // total = initial value => 0; 0 + 45 = 45

    // Flatten an array

    const nested = [[1, 2], [3, 4], [5]];

    const flat = nested.reduce((acc, val) => acc.concat(val), []);
    console.log(flat);

    // The .concat() method in JavaScript is used to combine (or merge) arrays or strings.
    // It returns a new array or string without modifying the original.

    // count frequency

    const letters = ["a", "b", "a", "b", "c"];

    const freq = letters.reduce((acc, char) => {
      acc[char] = (acc[char] || 0) + 1; // if char exits = 1,  otherwise o
      return acc; // { a: 1 } thats why acc[char]
    }, {});
    console.log(freq); // {a: 2, b: 2, c: 1}

    /*
      acc = {} ← empty object (starting value)

      Looping through: 'a', 'b', 'a', 'c', 'b' or

      Iterate over: ['a', 'a', 'a', 'c', 'b']

      acc['a'] = (acc['a'] || 0) + 1; // undefined → 0 + 1 = 1
          acc = { a: 1 }

      acc['a'] = (acc['a'] || 0) + 1; // 1 + 1 = 2
            acc = { a: 2 }
    */

    /* forEach, map, filter, reduce */

    // this formate undefined bcz forEach does not return anything
    // const nuls = [1, 2, 3, 4, 5];
    // console.log(nuls.forEach((n, i) => (n, i)));

    const nuls = [1, 2, 3, 4, 5];

    // forEach - does something
    nuls.forEach((n, i) => console.log(n, i));

    // map - transforms
    const squared = nuls.map((n) => n * n);
    console.log(squared);

    // filter - selects
    const even = nuls.filter((n) => n % 2 === 0);
    console.log(even);

    // reduce - turn anything into single value

    const total = nuls.reduce((sum, num) => sum + num * 5, 0);
    console.log(total);
  </script>
</html>