Category: JavaScript Answers

To set focus on an element in an HTML form using JavaScript, we call the input’s focus method.

For instance, we write

<input type="text" id="myText" />

to add an input.

Then we write

document.getElementById("myText").focus();

to select the input with getElementById.

Then we call focus to set focus on it.

To use moment.js to convert date to string in mm/dd/yyyy format with JavaScript, we use the format method.

For instance, we write

const startDate = moment().format("MM/DD/YYYY HH:mm:ss");

to create a moment object with moment with the current datetime.

Then we call format with a format string to return a string in mm/dd/yyyy format with the time in hours:minutes:seconds format.

To read in a local csv file in JavaScript, we can use the FileReader constructor.

For instance, we write:

<input type="file">

to add a file input.

Then we write:

const input = document.querySelector('input')
const fileReader = new FileReader()
fileReader.onload = (e) => {
  console.log(e.target.result)
}

input.onchange = (e) => {
  const [file] = e.target.files
  fileReader.readAsBinaryString(file)
}

to select the input with querySelector.

Next, we create a FileReader instance.

Then we set its onload property to a function that logs the file content.

Next, we set input.onchange to a function that gets the selected file from e.target.files.

And then we call fileReader.readAsBinaryString to read the file.

To create a unique array of objects with JavaScript, we use the array filter method.

For instance, we write

const getUniqueBy = (arr, prop) => {
  const set = new Set();
  return arr.filter((o) => !set.has(o[prop]) && set.add(o[prop]));
};

to define the getUniqueBy function.

In it, we create a set with the Set constructor.

And then we return the array returned by filter that adds the o[prop] to the set if it doesn’t exist and if the property value doesn’t exist.

To create a new file with JavaScript, we use the File constructor.

For instance, we write

const f = new File([""], "filename");

to call File with an array with the file contents and a string with the filename respectively.

We can pass in a third argument with some options.

For example, we write

const f = new File([""], "filename.txt", {
  type: "text/plain",
  lastModified: date,
});

to call File with an object with the MIME type and the lastModified date of the file as the 3rd argument.