Category: JavaScript Answers

To flatten a JavaScript object keys and values to a single depth array, we recursively put items into a new object with all the nested properties in the top level.

For instance, we write

const flattenObj = (obj, parent, res = {}) => {
  for (const key of Object.keys(obj)) {
    const propName = parent ? parent + "." + key : key;
    if (typeof obj[key] === "object" && obj[key] !== -null) {
      flattenObj(obj[key], propName, res);
    } else {
      res[propName] = obj[key];
    }
  }
  return res;
};

to define the flattenObj function.

In it, we get the keys of the obj object with Object.keys.

Then we loop through the keys with a for-of loop.

Then we define the propName by combining the parent property path string with the key.

Then if obj[key] is an object and it’s not null, we call flattenObj with it.

Otherwise we set obj[key] as the value of res[propName].

After the loop is done, res is returned.

Conclusion

To flatten a JavaScript object keys and values to a single depth array, we recursively put items into a new object with all the nested properties in the top level.

Sometimes, we want to check for palindrome in JavaScript.

In this article, we’ll look at how to check for palindrome in JavaScript.

How to check for palindrome in JavaScript?

To check for palindrome in JavaScript, we check if the string and the reversed version of the string is the same.

For instance, we write

const checkPalindrome = (str) => {
  return str === str.split("").reverse().join("");
};

to define the checkPalindrome function.

In it, we get the reversed string with str.split("").reverse().join("").

And we check if it’s the same as the original str string with ===.

Conclusion

To check for palindrome in JavaScript, we check if the string and the reversed version of the string is the same.

Sometimes, we want to add thousand separator with HTML number input with JavaScript.

In this article, we’ll look at how to add thousand separator with HTML number input with JavaScript.

How to add thousand separator with HTML number input with JavaScript?

To add thousand separator with HTML number input with JavaScript, we use the autoNumeric library.

To install it, we add

<script src="https://unpkg.com/autonumeric"></script>

Then we write

<input type="text" value="1234.56" />

to add an input.

Then we write

const domElement = document.querySelector("input");
const anElement = new AutoNumeric(domElement);

to select the input with querySelector.

And we convert it to an input with the thousand separator in its value by calling the AutoNumeric constructor with domElement.

Conclusion

To add thousand separator with HTML number input with JavaScript, we use the autoNumeric library.

Sometimes, we want to fix Uncaught TypeError: Cannot read property ‘value’ of undefined with JavaScript.

In this article, we’ll look at how to fix Uncaught TypeError: Cannot read property ‘value’ of undefined with JavaScript.

How to fix Uncaught TypeError: Cannot read property ‘value’ of undefined with JavaScript?

To fix Uncaught TypeError: Cannot read property ‘value’ of undefined with JavaScript, we should make sure we get the value property of an element.

For instance, we write

const el1 = document.getElementsByName("username")[0];
const el2 = document.getElementById("i1");

to get the first element with name attribute set to username with document.getElementsByName("username")[0].

And we get the element with ID i1 with document.getElementById("i1").

Conclusion

To fix Uncaught TypeError: Cannot read property ‘value’ of undefined with JavaScript, we should make sure we get the value property of an element.

Sometimes, we want to instantiate a JavaScript class in another JavaScript file.

In this article, we’ll look at how to instantiate a JavaScript class in another JavaScript file.

How to instantiate a JavaScript class in another JavaScript file?

To instantiate a JavaScript class in another JavaScript file, we have to make sure the file with the class is loaded before we instantiate the class.

For instance, we write

<script src="file1.js"></script>
<script src="file2.js"></script>

to load file.js before we load file2.js.

file1.js has the class.

And then we can instantiate the class in file2.js.

Conclusion

To instantiate a JavaScript class in another JavaScript file, we have to make sure the file with the class is loaded before we instantiate the class.