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Overview

  • The .filter() method returns a new array that contains some of the items from the original array, based on the condition you specify.
  • JavaScript will take your callback function and call it for every single item in the array.
  • For the .filter() method, the result of the callback function matters. When it's true, the item will be included in the resulting array. Otherwise, it won't.
  • JavaScript cannot make a smart guess that the numbers array becomes the number parameter in your callback function. What it does is that it calls your callback function while giving a value for the first parameter that you specified.
  • Use the plural -> singular naming convention when using the .filter() method.

Methods learned

  • .find() -finds items.
  • .filter() - filters out items.
  • .map() - transforms an array into another one.
  • .includes() - returns a boolean
  • .

Return positive numbers in an array

function getPositiveTemperatures(temperatures) {
const positiveNumbers = temperatures.filter(function(temp) {
    return temp > 0;
});
return positiveNumbers

}

// Sample usage - do not modify
console.log(getPositiveTemperatures([-5, 12, 3])); // [12, 3]
console.log(getPositiveTemperatures([1, -3, -2, 4, 10])); // [1, 4, 10]

And returning negative numbers in an array

function getFreezingTemperatures(temperatures) {
const negativeNumbers = temperatures.filter(function(temp) {
    return temp < 0;
});
return negativeNumbers// [14, 11]
}
// Sample usage - do not modify
console.log(getFreezingTemperatures([-5, 12, 3])); // [-5]
console.log(getFreezingTemperatures([1, -3, -2, 4, 10])); // [-3, -2]

Difference between .filter() and .find()

.filter() always returns an array - even if it is empty .find() returns the first array item. Returns undefined if that item is not found.

return a specific item from the array

function getYear(years, searchYear) {
    const result = years.find(function(item) {
  return item === searchYear;
});
return result;
}

// Sample usage - do not modify
console.log(getYear([2019, 2020, 2021], 2020)); // 2020
console.log(getYear([2019, 2020, 2021], 1990)); // undefined

Return odd years

function getOddYears(years) {
    const result = years.filter(function(year) {
        if(year % 2 !== 0){
            return year
        }
    });
    return result;
}
//or, remembering the early return

function getOddYears(years) {
  return years.filter(function(year) {
    return year % 2 !== 0;
  });

Basic includes

function isAppUsed(apps, app) {
    return apps.includes(app)
}

Return string length from array

Gotta start learning how to simplify down the code

function getStringSizes(strings) {
    const toNumber = strings.map(function(string) {
    return string.length;
});
return toNumber;
}
//or 
function getStringSizes(strings) {
    return strings.map(function(string) {
        return string.length;
    })
}

Classroom Grade project​

/** @param {number[]} grades */
export function getNumberOfGrades(grades) {
    // TODO: return the number of grades
    return grades.length

}

/** @param {number[]} grades */
export function getSumGrades(grades) {
    // TODO: return the sum of all the grades
    let total = 0
    const all = grades.forEach(function(grade){
        return total += grade
    });
    return total


}

/** @param {number[]} grades */
export function getAverageValue(grades) {
    // TODO: return the average value of all grades (sum of all grades divided by the total number of grades)
    //get sum
    let averageGrades = getSumGrades(grades) / getNumberOfGrades(grades)
    return averageGrades;

}

/** @param {number[]} grades */
export function getPassingGrades(grades) {
    // TODO: return all passing grades (10 and above)
  return grades.filter(function(grade) {
    return grade >=10;
  });

}

/** @param {number[]} grades */
export function getFailingGrades(grades) {
    // TODO: return all failing grades (9 and below)
  return grades.filter(function(grade) {
    return grade <=9;
  });

}

/** @param {number[]} grades */
export function getRaisedGrades(grades) {
    // TODO: return all the grades raised by 1 (no grade should exceed 20)
    let gradePlus = 0;
    return grades.map(function(grade){
            if(grade !== 20){
        return grade + 1
    }
    return grade;
    });


}

Previewing the Fetch function

Returns all citizens that can vote

export function getVotersCount(ages) {
    console.log(ages);
  const newArray = ages.filter(function(age) {
    return age >= 18;
  });
  console.log(newArray.length) //not needed
  return newArray.length;
}

//or with chaining....
export function getVotersCount(ages) {
    return ages.filter(function(age){
        return age >= 18;
    }).length;
}

#Review​

  • The .filter() method returns a new array that contains some of the items from the original array, based on the condition you specify.
  • JavaScript will take your callback function and call it for every single item in the array.
  • For the .filter() method, the result of the callback function matters. When it's true, the item will be included in the resulting array. Otherwise, it won't.
  • JavaScript cannot make a smart guess that the numbers array becomes the number parameter in your callback function. What it does is that it calls your callback function while giving a value for the first parameter that you specified.
  • Use the plural -> singular naming convention when using the .filter() method.
  • The .find() method returns the first item which matches the condition that you specify. If no items were found, you will get back undefined.
  • The .filter() method always returns an array. Even if it's empty.
  • The array .map(callback) method allows you to transform an array into another one.
  • The array .includes(item) method takes an item and returns true when that item exists in the array and false otherwise.
  • The array .join(glue) method returns a string of the array elements separated by the glue.