Advanced Array Methods(reduce)

reduce 

:  Accepts a callback function and an optional second parameter

  Iterates through an array

  Runs a callback on each value in the array

  The first parameter to the callback is either the first value in the array or the optional second parameter

  The first parameter to the callback is often called "accumulator"

  The returned value from the callback becomes the new value of accumulator

Whatever is returned from the callback function, becomes the new value of the accumulator

/*

Write a function called extractValue which accepts an array of objects and a key and returns a new array with the value of each object at the key.

Examples:

    var arr = [{name: 'Elie'}, {name: 'Tim'}, {name: 'Matt'}, {name: 'Colt'}]

    extractValue(arr,'name') // ['Elie', 'Tim', 'Matt', 'Colt']

*/

function extractValue(arr, key){

    return arr.reduce(function(accumulator, name){

> acc.push(next[key]);

        return accumulator[key];    > return acc;

    })

}

/*

Write a function called vowelCount which accepts a string and returns an object with the keys as the vowel and the values as the number of times the vowel appears in the string. This function should be case insensitive so a lowercase letter and uppercase letter should count

Examples:

    vowelCount('Elie') // {e:2,i:1};

    vowelCount('Tim') // {i:1};

    vowelCount('Matt') // {a:1})

    vowelCount('hmmm') // {};

    vowelCount('I Am awesome and so are you') // {i: 1, a: 4, e: 3, o: 3, u: 1};

*/

function vowelCount(str){

   var vowels="aeiou";

   return str.toLowerCase().split("").reduce(function(acc, next){

       > if(vowels indexOf(next) !== -1){

               if(acc[next]){

acc[next]++;

} else {

acc[next] = 1;

       }

   }

return acc;

}, {});

}

/*

Write a function called addKeyAndValue which accepts an array of objects and returns the array of objects passed to it with each object now including the key and value passed to the function.

Examples:

    var arr = [{name: 'Elie'}, {name: 'Tim'}, {name: 'Matt'}, {name: 'Colt'}];

    

    addKeyAndValue(arr, 'title', 'Instructor') // 

      [

        {title: 'Instructor', name: 'Elie'}, 

        {title: 'Instructor', name: 'Tim'}, 

        {title: 'Instructor', name: 'Matt'}, 

        {title: 'Instructor', name: 'Colt'}

       ]

*/

function addKeyAndValue(arr, key, value){

    return arr.reduce(functions(acc, next, idx){

acc[idx][key] = value;

return acc;

}, arr);

}

/*

Write a function called partition which accepts an array and a callback and returns an array with two arrays inside of it. The partition function should run the callback function on each value in the array and if the result of the callback function at that specific value is true, the value should be placed in the first subarray. If the result of the callback function at that specific value is false, the value should be placed in the second subarray. 

Examples:   

    function isEven(val){

        return val % 2 === 0;

    }

    

    var arr = [1,2,3,4,5,6,7,8];

    

    partition(arr, isEven) // [[2,4,6,8], [1,3,5,7]];

    

    function isLongerThanThreeCharacters(val){

        return val.length > 3;

    }

    

    var names = ['Elie', 'Colt', 'Tim', 'Matt'];

    

    partition(names, isLongerThanThreeCharacters) // [['Elie', 'Colt', 'Matt'], ['Tim']]

*/

function partition(arr, callback){

    return arr.reduce(function(acc, next){

if(callback(next)){

acc.[0].push(next);

} else {

acc.[1].push(next);

}

return acc;

}, [[], []]);

}