Category: JavaScript Answers

Sometimes, we want to select all checkboxes using jQuery

In this article, we’ll look at how to select all checkboxes using jQuery.

Select All Checkboxes Using jQuery

To select all checkboxes using jQuery, we can select all the checkbox input elements.

Then we call prop to set the checked attribute of each checkbox to a given value.

For instance, we write:

<p id="checkBoxes">  
  <input type="checkbox" class="checkBoxClass" id="Checkbox1" />  
  <br />  
  <input type="checkbox" class="checkBoxClass" id="Checkbox2" />  
  <br />  
  <input type="checkbox" class="checkBoxClass" id="Checkbox3" />  
  <br />  
  <input type="checkbox" class="checkBoxClass" id="Checkbox4" />  
  <br />  
  <input type="checkbox" class="checkBoxClass" id="Checkbox5" />  
  <br />  
</p>

to add the checkboxes.

Then we write:

$(".checkBoxClass").prop('checked', true);

to call the prop method on all the checkboxes with class checkBoxClass and set their checked attributes to true .

Therefore, all the checkboxes will be checked.

Conclusion

To select all checkboxes using jQuery, we can select all the checkbox input elements.

Then we call prop to set the checked attribute of each checkbox to a given value.

Sometimes, we want to get the width and height of an image with file reader.

In this article, we’ll look at how to get the width and height of an image with file reader.

Get the Width and Height of an Image with File Reader

To get the width and height of an image with file reader, we can create a new img element object with the Image constructor.

For instance, we can write:

<input type='file'>

to add the file input.

Then we write:

const reader = new FileReader();
reader.onload = (theFile) => {
  const image = new Image();
  image.src = theFile.target.result;
  image.onload = () => {
    console.log(image.width, image.height);
  };
};

const input = document.querySelector('input')
input.addEventListener('change', e => {
  const [file] = e.target.files
  reader.readAsDataURL(file);
})

We create a new FileReader instance and assign it to reader .

Then we set reader.onload to a function that creates a new Image instance.

Next, we set the src property of the Image instance to theFile.target.result , which has the result that’s read from the readAsDataURL method.

Then we set the image.onload property to a function that gets the width and height of the image.

After that, we get the input with document.querySelector .

And then we call addEventListener on it to add a change event listener.

In the event handler, we get the file object from e.target.files via destructuring.

Finally, we call reader.readAsDataURL with file to read the selected file to get the size.

Conclusion

To get the width and height of an image with file reader, we can create a new img element object with the Image constructor.

Sometimes, we want to convert a canvas’ content to a PDF with JavaScript.

In this article, we’ll look at how to convert a canvas’ content to a PDF with JavaScript.

Convert a Canvas’ Content to a PDF with JavaScript

To convert a canvas’ content to a PDF with JavaScript, we can use the jsPDF library.

For instance, we can write:

<script src="https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.3.1/jspdf.umd.min.js"></script>

<canvas id="myCanvas" width="200" height="200"></canvas>

<button>
  download
</button>

to add the jsPDF library with a script tag.

Then we add the canvas elements and button to download the PDF.

Next, we write:

const canvas = document.querySelector('canvas');
const download = document.querySelector('button');
const context = canvas.getContext('2d');
const {
  jsPDF
} = window.jspdf;
const pdf = new jsPDF();

context.fillStyle = 'yellow';
context.fillRect(0, 0, 100, 100);

download.addEventListener("click", () => {
  const imgData = canvas.toDataURL("image/jpeg", 1.0);
  pdf.addImage(imgData, 'JPEG', 0, 0);
  pdf.save("download.pdf");
}, false);

to get the canvas and the button with document.querySelector .

Then we get the canvas context with canvas.getContext .

Next, we create a new jsPDF instance with:

const {
  jsPDF
} = window.jspdf;
const pdf = new jsPDF();

Then we draw a yellow square with:

context.fillStyle = 'yellow';
context.fillRect(0, 0, 100, 100);

Next, we add a click handler to the button with:

download.addEventListener("click", () => {
  const imgData = canvas.toDataURL("image/jpeg", 1.0);
  pdf.addImage(imgData, 'JPEG', 0, 0);
  pdf.save("download.pdf");
}, false);

In the event handler, we call canvas.toDataURL with 'image/jpeg' to return the image data version of the canvas.

Then we pass that into pdf.addImage with 'JPEG' to add the image into the PDF.

Finally, we call pdf.save with the file name of the PDF.

Now when we click download, we see a PDF with a yellow square.

Conclusion

To convert a canvas’ content to a PDF with JavaScript, we can use the jsPDF library.

Sometimes, we want to listen to the input event with jQuery.

In this article, we’ll look at how to listen to the input event with jQuery.

Listen to the input Event with jQuery

To listen to the input event with jQuery, we can call the on method with 'input' as the first argument.

For instance, we write:

<textarea></textarea>
<div>

</div>

to add the textarea and the div to display the inputted value.

Then we write:

$('textarea').on('input', () => {
  const text = $('textarea').val();
  $('div').html(text);
});

to call on with 'input' with a callback to get the textarea ‘s value with:

$('textarea').val()

and assign it to text .

Then we get the text and put it in the div with:

$('div').html(text)

Conclusion

To listen to the input event with jQuery, we can call the on method with 'input' as the first argument.

Sometimes, we want to find all subsets of a set in JavaScript.

In this article, we’ll look at how to find all subsets of a set in JavaScript.

Find All Subsets of a Set in JavaScript

To find all subsets of a set in JavaScript, we can use the reduce method to get all subsets of a set.

For instance, we can write:

const getAllSubsets =
  theArray => theArray.reduce(
    (subsets, value) => subsets.concat(
      subsets.map(set => [value, ...set])
    ),
    [
      []
    ]
  );

console.log(getAllSubsets([1, 2, 3]));

to create the getAllSubsets function that takes the theArray array parameter.

We then call reduce on it with a callback that takes the subsets and value parameters.

We callsubsets.concat with the array created by subsets.map with a callback that takes the value and set entries and combine them into a new subset.

The 2nd argument is an array with an empty array in it, which is the initial value of subsets .

Therefore, when we call getAllSubsets with [1, 2, 3] , we get:

[]
[1]
[2]
[2, 1]
[3]
[3, 1]
[3, 2]
[3, 2, 1]

in the array.

Conclusion

To find all subsets of a set in JavaScript, we can use the reduce method to get all subsets of a set.