Category: JavaScript Answers

The JavaScript standard library has a Math object, which has a random method to generate random numbers between 0 and 1. It takes no arguments.

By itself, it’s of limited use since it can only generate numbers between 0 and 1, but we can easily use it to generate random numbers in any range.

For example, if we want to generate a number between a minimum and a maximum number, we can write the following code:

const min = 1;
const max = 100;
const num = Math.random() * (max - min) + min;
console.log(num);

In the code above, we have a formula that has the min number as the minimum number, then we add Math.random() * (max — min) to min. max-min would be positive since max is bigger than min and Math.random() returns a number between 0 and 1 so it would either be 0 or positive.

If max is the same as min, then we get min as the result of num, which is the lowest number we’ll accept, and if Math.random() is 0 we get the same thing.

If Math.random() is 1, we get max — min + min which is the same as max. This means that the formula would never be outside of the number range between min and max, which is what we want. The formula above will get us any floating-point number. If want integer results only, we can write:

let min = 1;
let max = 10;
min = Math.ceil(min);
max = Math.floor(max);
const num = Math.floor(Math.random() * (max - min)) + min;
console.log(num);

In the code above, we rounded down our result to the nearest integer with the Math.floor(Math.random() * (max — min)) method call. This makes sure the result generated would be between 1 inclusive and 10 exclusive.

To generate a number from a minimum to a maximum number, we can do it in a few ways. If we want each entry to increment by 1, we can use the Array.from method. The Array.from method takes an object which has the length property.

To generate a number within a range, like from 1 to 10, we can make a constant for the maximum number and another one for the minimum number. Then the length would be the maximum minus the minimum plus one.

We need to add one since the maximum minus the minimum is one less than the length we want. The second argument of the Array.from method is a function that lets us map the values to the ones we want.

The first parameter is the value of the original array since we didn’t pass in an array into the first argument, this parameter isn’t useful for us.

The second argument is the index of the array, which will range from 0 to the length which we specified as the length property minus 1. So if we want to generate an array of numbers from 1 to 10, we can write:

const max = 10;
const min = 1;
const arr = Array.from({
  length: max - min + 1
}, (v, i) => min + i);
console.log(arr)

In the code above, we specified the length property of the array that we’ll generate, which is max — min + 1 or 10 – 1+1, which is 10. That’s the length we want. In the second argument, we have a function that maps the index i into min + i, which is will be 1 + 0 , 1 + 1 , 1 + 2 , …, up to 1 + 9 . Then if we run the console.log statement in the last line of the code above, we get:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

It’s easy to generate different kinds of number arrays by adjusting the function in the second parameter since it maps to any number we want. To generate the first n odd numbers, we can write:

const arr = Array.from({
  length: 10
}, (v, i) => 2 * i + 1);
console.log(arr)

In the code above, we have a function that generates odd numbers from a formula. 2 * i + 1 always generates an odd number since 2 times any integer is an event number, so if we add 1 to it, then it will become an odd number. It’s useful for any number sequence that has a pattern that can be put into a formula or be expressed in terms of conditionals or other flow control statements.

Moment.js is a popular JavaScript date and time manipulation library that we can use to calculate various things with date and time.

In this library, we’ll look at how to get the hours difference between 2 dates with moment.js

The moment.duration Method

The moment.duration lets us calculate the duration between 2 dates.

We can combine it with the diff method to calculate the hours difference between 2 dates.

For instance, we can write:

const startTime = moment('2021-01-01')  
const end = moment('2021-02-01')  
const duration = moment.duration(end.diff(startTime));  
const hours = duration.asHours();  
console.log(hours)

We call end.diff(startTime) to calculate the difference between end and startTime .

Then we call moment.duration to get the moment duration.

And finally, we call asHours to get the duration as hours.

Therefore, hours is 744 hours.

Moment.js fromNow and from Methods

We can use the fromNow method to get a human readable string of the difference between a date and now.

For instance, we can write:

const diff = moment('2021-01-01').fromNow()   
console.log(diff)

And we get something like ‘2 months ago’ .

We can use moment.js from method to get a human-readable string between the difference between 2 dates.

For instance, we can write:

const a = moment('2021-01-01');  
const b = moment('2021-02-01');  
const diff = a.from(b);  
console.log(diff)

And we get 'a month ago’ as a value of diff since February 1, 2021 is 1 month ahead of January 1, 2021.

Moment.js diff Method with Unit as the Second Argument

We can pass in a second argument to the diff method to return the difference in the unit specified.

For instance, we can write:

const a = moment('2021-01-01');  
const b = moment('2021-02-01');  
const diff = a.diff(b, 'hours');  
console.log(diff)

We get the difference between date-time a and b .

And a is behind b by 1 month, so we get -744 as the result of diff .

The 2nd argument is 'hours' so the result is in hours.

Conclusion

We can get the difference between 2 dates with the units we specified with various moment.js methods.

Also, we can get human readable durations with moment.js.

Sometimes, we want to detect whether an HTML element’s size has changed.

In this article, we’ll look at how to detect an HTML element that has changed in size.

ResizeObserver

The ResizeObserver API lets us watch for changes in the size of an HTML element.

For instance, we can write:

const div = document.querySelector('div')
const sleep = ms => new Promise((resolve) => setTimeout(resolve, ms));

(async () => {
  for (let i = 0; i < 10; i++) {
    const p = document.createElement('p')
    p.textContent = 'hello'
    div.appendChild(p)
    await sleep(1000)
  }
})();

new ResizeObserver(e => {
  const [{
    borderBoxSize,
    contentBoxSize,
    contentRect,
    devicePixelContentBoxSize: [devicePixelBoxSize]
  }] = e;
  console.log(borderBoxSize, contentBoxSize, contentRect, devicePixelBoxSize)
}).observe(div);

We have a div that we want to watch for size changes.

And we have an async function that adds a p element to the div to change its size every second.

Then we use the ResizeObserver with a callback to watch for size changes.

We call observe to with the div element object to watch for size changes of the div.

In the ResizeObserver callback, we get an event object in the parameter that has info about the element being watched.

We get a few properties in the e object.

The borderBoxSize property lets us get the border-box size of the observed element.

The contentBoxSize property lets us get the content box size of the observed element.

The contentRect property lets us get the new size of the observed element.

The devicePixelContentBoxSize property has the new content-box size in device pixels of the observed element.

These properties are all objects.

borderBoxSize , contentBoxSize , and contentRect properties has the blockSize and inlineSize properties which has the size in pixels depending on whether the element is a block or inline element.

contentRect has the coordinates of the rectangle with the left , right , top and bottom properties.

left and top have the coordinates of the x and y top-left corner in pixels.

right and bottom have the coordinates of the x and y bottom-right corner in pixels.

x and y also have the coordinates of the x and y top-left corner in pixels.

Conclusion

We can use the ResizeObserver API to watch for size changes of a given HTML element.

Sometimes, we may want to watch for changes in the DOM in our web app.

In this article, we’ll look at how to watch for changes in the DOM with JavaScript.

MutationObserver

One way to watch for DOM changes in our JavaScript web app is to use the MutationObserver constructor.

For instance, we can write:

const observer = new MutationObserver((mutations, observer) => {
  console.log(mutations, observer);
});

observer.observe(document, {
  subtree: true,
  attributes: true
});

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

(async () => {
  for (let i = 0; i < 5; i++) {
    const p = document.createElement('p')
    p.textContent = 'hello'
    document.body.appendChild(p)
    await sleep(1000)
  }
})();

to create the MutationObserver instance with a loop to append child elements to the body within the async function.

We insert a new p element after 1 second 5 times.

We pass in a callback into the MutationObserver constructor that runs when the DOM changes.

Then we call observe on the element that we want to observe changes for.

subtree set to true means that we watch for child element changes.

attributes set to true means we watch for element attribute changes.

Other options include:

• childList — set to true to observe the target’s children
• characterData— set to true to observe the target’s data
• attributeOldValue— set to true to observe the element’s attribute’s value before the DOM change
• characterDataOldValue— set to true to observe the target’s character data before a change is made
• attributeFilter — set to the attribute’s local names to be observed.

The mutations parameter has a bunch of properties that have the changes applied.

The mutations.target property has the target element that’s changed.

mutations.target.lastChild has the bottommost child node in the element being watched.

mutations.target.lastElementChild has the bottommost child element node in the element being watched.

Listen to the DOMSubtreeModified Event

Another way to listen for DOM strucuter changes is to listen to the DOMSubtreeModified event.

For instance, we can write:

document.addEventListener('DOMSubtreeModified', (e) => {
  console.log(e)
})

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

(async () => {
  for (let i = 0; i < 5; i++) {
    const p = document.createElement('p')
    p.textContent = 'hello'
    document.body.appendChild(p)
    await sleep(1000)
  }
})();

to add an event listener for the document’s DOMSubtreeModified event.

The e parameter is an MutationEvent object.

The e.target property has the element that’s changed.

e.path has the path to the element that’s changed as an array of elements leading to the changed element.

e.children has an HTMLCollection object with the elements changed.

Conclusion

We can use the MutationObserver and the DOMSubtreeModified event to listen for changes to the DOM.