Go through arrays in JavaScript (Part III)

Hey guys , welcome to Omkar's blogs .

Today we talk about some more array methods. Here are some array methods -

• reduce()
• filter()

reduce()

The reducer() method execute a reducer function on each element of an array to return single output value.

const numList = [1, 2, 3, 4, 5, 6, 7, 8, 9];

// function to add each elements
function addList(accumulator, currentValue) {
  return accumulator + currentValue;
}

// reduce add each element of the array
let addNumList = numList.reduce(addList);
console.log(addNumList);

// Output: 45

reducer() syntax arr.reduce(callback(accumulator, currentValue), initialValue)

In reducer() method , we can pass two parameters -

• First parameter is callback function which execute on each array elements . In callback function, accumulator stores callback return values and current element passed from array store in currentValue.
• Second parameter is initialValue , which is optional parameter. This value will be pass to callback on first time when callback is call. The first element acts as the accumulator on the first call and callback() won't execute on it.

reduce() does not change the original array.

filter()

The filter() method return the new array with values that satisfies the conditions given by function.

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9];

// function to check even numbers
function checkEven(number) {
  if (number % 2 == 0)
    return true;
  else
    return false;
}

// create a new array by filter even numbers from the array
let evenNumbers = numbers.filter(checkEven);
console.log(evenNumbers);

// Output: [ 2, 4, 6, 8]

filter() syntax arr.filter(callback(element), thisArg)

In filter() method , we can pass two parameters -

• First parameter is a callback function which executes on each array elements and return boolean values (true , false).
• Second parameter is optional parameter thisArg . By default this value is undefined. This value is use as this when executing callback.

filter() method return new array with values which satisfies conditions given by function.