map, filter and reduce methods were introduced in ES5. These methods are applied to arrays.

✨Map()

map method in javascript returns a new array depending on the condition applied to the current array for each element. It doesn't change the original array.

Syntax: newArray = (originalArray).map(callback);

Here, callback is a function that takes 3 parameters - currentValue, currentValueIndex, originalArray . The last two parameters are optional.

Let's say we have an array of objects - nameArray and we want to modify each object by adding a new field - id .

let nameArray = [
    {firstName: "Walter", lastName: "White"},
    {firstName: "Tony", lastName: "Stark"}
]
let count=1;
newNameArray = nameArray.map((currentObject) => {  
    // currentObject = {firstName: "Walter", lastName: "White"}
    return {id: count++, ...currentObject}; // return modified object
});
console.log(newNameArray);

// OUTPUT:-
// [
//  {id: 1, firstName: "Walter", lastName: "White"},
//  {id: 2, firstName: "Tony", lastName: "Stark"}
// ]

✨Filter()

filter method in javascript filters out elements from the original array which doesn't fulfill the condition provided by a function. This method also returns a new array and doesn't change the original array.

Syntax: newArray = (originalArray).filter(callback);

Same as map(), callback takes 3 parameters - currentValue, currentValueIndex, originalArray . The last two parameters are optional.

Let's say we have an array of objects nameArray and we want to filter out objects with even id .

let nameArray = [
    {id: 1, firstName: "Walter", lastName: "White"},
    {id: 2, firstName: "Tony", lastName: "Stark"},
    {id: 3, firstName: "Rebecca", lastName: "Pearson"}
]
newNameArray = nameArray.filter((currentObject) => {  
    // currentObject = {id:1, firstName: "Walter", lastName: "White"}
    return currentObject.id%2 != 0;// returns true, hence not removed
});
console.log(newNameArray);

// OUTPUT:-
// [
//  {id: 1, firstName: "Walter", lastName: "White"},
//  {id: 3, firstName: "Rebecca", lastName: "Pearson"}
// ]

✨Reduce()

reduce method in javascript runs a "reducer" callback function on each element of an array. It does not change the original array.

Syntax: newArray = originalArray.reduce(callback, initialValue);

callback: function(previousValue, currentValue, currentIndex, OriginalArray)

Here, the callback function takes 4 parameters - previousValue, currentValue , currentIndex and OriginalArray . The last two are optional.

If initialValue (optional) is not given then the first element is assigned to previousValue and the iteration starts from the next element. If we supply initialValue then it will be assigned to previousValue .

✨Ways to use map and filter methods on the same array:-

Let's say, from the above example, we want to get all names as firstName+lastName for odd id .

1. We can simply apply map and filter methods separately using extra arrays.

    let nameArray = [
        {id: 1, firstName: "Walter", lastName: "White"},
        {id: 2, firstName: "Tony", lastName: "Stark"},
        {id: 3, firstName: "Rebecca", lastName: "Pearson"}
    ]
    // filters out objects with even id
    filteredArray = nameArray.filter(currentObject => 
                             currentObject.id%2 != 0)
   
    modifiedArray = filteredArray.map(currentObject => 
             currentObject.firstName+" "+currentObject.lastName)
   
    console.log(filteredArray);
    console.log(modifiedArray);
    // OUTPUT:
    // [
    //  { id: 1, firstName: 'Walter', lastName: 'White' },
    //  { id: 3, firstName: 'Rebecca', lastName: 'Pearson' }
    // ]
    // [ 'Walter White', 'Rebecca Pearson' ]
   

2. Simplify the above code by removing the use of another array to store values.

   nameArray.filter(currentObject => currentObject.id%2 != 0) will return an array with filtered objects which will then be used by map method.

    let nameArray = [
        {id: 1, firstName: "Walter", lastName: "White"},
        {id: 2, firstName: "Tony", lastName: "Stark"},
        {id: 3, firstName: "Rebecca", lastName: "Pearson"}
    ]
    modifiedArray = nameArray.filter(currentObject => 
        currentObject.id%2 != 0).map(currentObject =>
        currentObject.firstName+" "+currentObject.lastName)
   
    console.log(modifiedArray);
    // OUTPUT:
    // [ 'Walter White', 'Rebecca Pearson' ]
   

3. Use reduce method.

    let nameArray = [
        {id: 1, firstName: "Walter", lastName: "White"},
        {id: 2, firstName: "Tony", lastName: "Stark"},
        {id: 3, firstName: "Rebecca", lastName: "Pearson"}
    ]
   
    reducedArray = nameArray.reduce((filteredArray, currentObject) => 
    {
      if (currentObject.id%2 != 0) {
         let modifiedValue = currentObject.firstName+" "+ 
                             currentObject.lastName
         filteredArray.push(modifiedValue);
      }
      return filteredArray;
    }, []); // initialvalue as empty array
   
    console.log(reducedArray)
    // OUTPUT:
    // [ 'Walter White', 'Rebecca Pearson' ]
   

   For previousValue , we have taken filteredArray as a variable and assigned [] to make it an array. Every time the currentObject satisfies the condition, it is pushed to the filteredArray .The returned value is again assigned to filteredArray and the same goes on for the other element.

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Happy Learning! 🍀