Fundamentals 編

JSchallenger にチャレンジ - Fundamentals / Basics
JSchallenger にチャレンジ - Fundamentals / Arrays
JSchallenger にチャレンジ - Fundamentals / Objects
JSchallenger にチャレンジ - Fundamentals / Dates
JSchallenger にチャレンジ - Fundamentals / Sets

DOM 編

JSchallenger にチャレンジ - DOM / DOM selector methods
JSchallenger にチャレンジ - DOM / Events and user interactions
JSchallenger にチャレンジ - DOM / DOM manipulation
JSchallenger にチャレンジ - DOM / DOM fundamentals
JSchallenger にチャレンジ - DOM / Recursive functions

cyamy 2022/07/11に更新

Accessing object properties one

Write a function that takes an object with two properties as argument
It should return the value of the property with key country

回答

function myFunction(obj) {   
   return obj.country
}

cyamy 2022/07/11に更新

Accessing object properties two

Write a function that takes an object with two properties as argument
It should return the value of the property with key 'prop-2'
Tipp: you might want to use the square brackets property accessor

回答

function myFunction(obj) {
  return obj['prop-2']
}

cyamy 2022/07/11に更新

Accessing object properties three

Write a function that takes an object with two properties and a string as arguments
It should return the value of the property with key equal to the value of the string

回答

function myFunction(obj, key) {
  return obj[key]
}

cyamy 2022/07/11に更新

Check if property exists in object

Write a function that takes an object (a) and a string (b) as argument
Return true if a has a property with key b
Return false otherwise

回答

function myFunction(a, b) {
   return b in a;
}

cyamy 2022/07/11に更新

Creating Javascript objects one

Write a function that a string (a) as argument
Create an object that has a property with key 'key' and a value of a
Return the object

回答

function myFunction(a) {
   return { key: a };
}

cyamy 2022/07/11に更新

Creating Javascript objects two

Write a function that takes two strings (a and b) as arguments
Create an object that has a property with key 'a' and a value of 'b'
Return the object

回答

function myFunction(a, b) {
   return { [a]: b };
}

cyamy 2022/07/11に更新

Creating Javascript objects three

Write a function that takes two arrays (a and b) as arguments
Create an object that has properties with keys 'a' and corresponding values 'b'
Return the object

回答

function myFunction(a, b) {
  return a.reduce((prev, current, index) => {
    return {
       ...prev,
      [current]: b[index]
    }
  },{})
}

cyamy 2022/07/11に更新

Extract keys from Javascript object

Write a function that takes an object (a) as argument
Return an array with all object keys

function myFunction(a) {
  return Object.keys(a);
}

cyamy 2022/07/21に更新

Sum object values

Write a function that takes an object (a) as argument
Return the sum of all object values

回答

function myFunction(a) {
  return Object.values(a).reduce((prev, current) => {
    return prev + current;
  },0)
}

cyamy 2022/07/11に更新

Remove a property from an object

Write a function that takes an object as argument
It should return an object with all original object properties
except for the property with key 'b'

回答

function myFunction(obj) {
  const {b, ...param} = obj;
  return param;
}

cyamy 2022/07/11に更新

Merge two objects with matching keys

Write a function that takes two objects as arguments
Unfortunately, the property 'b' in the second object has the wrong key
It should be named 'd' instead
Merge both objects and correct the wrong property name
Return the resulting object
It should have the properties 'a', 'b', 'c', 'd', and 'e'

回答

function myFunction(x, y) {
  const { b, ...param } = y;
  return { ...x, ...param, d: b };
}

cyamy 2022/07/11に更新

Multiply all object values by x

Write a function that takes an object (a) and a number (b) as arguments
Multiply all values of 'a' by 'b'
Return the resulting object

回答

function myFunction(a, b) {
  return Object.entries(a).reduce((prev, current) => {
    const [key, value] = [...current];
    return {
      ...prev,
      [key]: value * b,
    };
  }, {});
}

cyamy 2022/07/11に更新

Swap object keys and values

Write a function that takes an object as argument
Somehow, the properties and keys of the object got mixed up
Swap the Javascript object's key with its values and return the resulting object

回答

function myFunction(obj) {
  return Object.entries(obj).reduce((prev, current) => {
    const [key, value] = [...current];
    return {
      ...prev,
      [value]: key,
    };
  }, {});
}

cyamy 2022/07/11に更新

Replace empty strings in object with null values

Write a function that takes an object as argument
Some of the property values contain empty strings
Replace empty strings and strings that contain only whitespace with null values
Return the resulting object

回答

function myFunction(obj) {
  return Object.entries(obj).reduce((prev, current) => {
    const [key, value] = [...current];
    return {
      ...prev,
      [key]: value.trim() === '' ? null : value,
    };
  }, {});
}

cyamy 2022/07/11に更新

Extracting information from objects

Write a function that takes an object as argument containing properties with personal information
Extract firstName, lastName, size, and weight if available
If size or weight is given transform the value to a string
Attach the unit cm to the size
Attach the unit kg to the weight
Return a new object with all available properties that we are interested in

回答

function myFunction(obj) {
  return {
    fn: obj.fn,
    ln: obj.ln,
    ...(obj.size && { size: `${obj.size}cm` }),
    ...(obj.weight && { weight: `${obj.weight}kg` }),
  };
}

cyamy 2022/07/11に更新

Add property to each object in array

Write a function that takes an array of objects and a string as arguments
Add a property with key 'continent' and value equal to the string to each of the objects
Return the new array of objects
Tipp: try not to mutate the original array

回答

function myFunction(arr, str) {
  return arr.map((obj) => ({ ...obj, continent: str }));
}

cyamy 2022/07/11に更新

Convert array to object with counter

Write a function that takes an array of numbers as argument
Convert the array to an object
It should have a key for each unique value of the array
The corresponding object value should be the number of times the key occurs within the array

回答

function myFunction(a) {
  return a.reduce((prev, current) => {
    return {
      ...prev,
      [current]: (prev[current] ?? 0) + 1,
    };
  }, {});
}

cyamy 2022/08/20に更新

Check if property exists in object and is truthy

Write a function that takes an object (a) and a string (b) as argument
Return true if the object has a property with key 'b', but only if it has a truthy value
In other words, it should not be null or undefined or false
Return false otherwise

回答

function myFunction(a, b) {
   return !!a[b]
}

cyamy 2022/10/05

Return nested object property

Write a function that takes an object as argument
In some cases the object contains other objects with some deeply nested properties
Return the property 'b' of object 'a' inside the original object if it exists
If not, return undefined

回答

function myFunction(obj) {
    return obj.a?.b;
}