Category: JavaScript Answers

Sometimes, we want to draw arrow on canvas tag with JavaScript.

In this article, we’ll look at how to draw arrow on canvas tag with JavaScript.

How to draw arrow on canvas tag with JavaScript?

To draw arrow on canvas tag with JavaScript, we use the moveTo and lineTo methods.

For instance, we write

const canvasArrow = (context, fromx, fromy, tox, toy) => {
  const headlen = 10;
  const dx = tox - fromx;
  const dy = toy - fromy;
  const angle = Math.atan2(dy, dx);
  context.moveTo(fromx, fromy);
  context.lineTo(tox, toy);
  context.lineTo(
    tox - headlen * Math.cos(angle - Math.PI / 6),
    toy - headlen * Math.sin(angle - Math.PI / 6)
  );
  context.moveTo(tox, toy);
  context.lineTo(
    tox - headlen * Math.cos(angle + Math.PI / 6),
    toy - headlen * Math.sin(angle + Math.PI / 6)
  );
};

const ctx = document.getElementById("c").getContext("2d");
ctx.beginPath();
canvasArrow(ctx, 10, 30, 200, 150);
ctx.stroke();

to call moveTo to move to the starting point iun the canvasArrow function.

Then we call lineTo to draw to the next points.

Finally, we call stroke to finishing drawing.

Conclusion

To draw arrow on canvas tag with JavaScript, we use the moveTo and lineTo methods.

Sometimes, we want to delete or remove nodes on Firebase and JavaScript.

In this article, we’ll look at how to delete or remove nodes on Firebase and JavaScript.

How to delete or remove nodes on Firebase and JavaScript?

To delete or remove nodes on Firebase and JavaScript, we use the remove method.

For instance, we write

ref.child(key).remove();

to get the child node with the given key with ref.child(key).

And then we call remove to remove it.

Conclusion

To delete or remove nodes on Firebase and JavaScript, we use the remove method.

Sometimes, we want to change the Chart.js axes label font size with JavaScript.

In this article, we’ll look at how to change the Chart.js axes label font size with JavaScript.

How to change the Chart.js axes label font size with JavaScript?

To change the Chart.js axes label font size with JavaScript, we set the fontSize property.

For instance, we write

const options = {
  scales: {
    yAxes: [
      {
        ticks: {
          fontSize: 40,
        },
      },
    ],
  },
};

to set the ticks.fontSize property in the yAxes array to set the font size of the y-axis labels.

Conclusion

To change the Chart.js axes label font size with JavaScript, we set the fontSize property.

Sometimes, we want to write regex to match stuff between parentheses with JavaScript.

In this article, we’ll look at how to write regex to match stuff between parentheses with JavaScript.

How to write regex to match stuff between parentheses with JavaScript?

To write regex to match stuff between parentheses with JavaScript, we put a parenthesis between the parentheses.

For instance, we write

/\(([^()]+)\)/g

to put ([^()]+ in the parentheses to match parentheses in a string.

Conclusion

To write regex to match stuff between parentheses with JavaScript, we put a parenthesis between the parentheses.

Sometimes, we want to validate select box with JavaScript.

In this article, we’ll look at how to validate select box with JavaScript.

How to validate select box with JavaScript?

To validate select box with JavaScript, we check if the select element’s value property is set.

For instance, we write

<select id="select" required="required">
  <option value="">Choose an option</option>
  <option value="option1">Option1</option>
  <option value="option2">Option2</option>
  <option value="option3">Option3</option>
</select>

to add a select drop down.

Then we write

const select = document.getElementById("select");
if (select.value) {
  // ...
}

to select the select element with getElementById.

Then we check for the select element’s value with select.value.

Conclusion

To validate select box with JavaScript, we check if the select element’s value property is set.