/* Code Comments */

Scroll Events And Invoking Lodash’s `_.throttle` Or `_.debounce`

May 06, 2019

Recently, I was working on a project where I wanted the scroll event to invoke a function that would affect other elements on the page. For performance reasons, however, I wanted to throttle the function calls so as to not kill performance with costly calculations on every scroll event.

Getting the throttling to actually occur proved to be a challenge. Instead, it was as if my function was being swallowed up and never firing at all. It turns out, it wasn’t — I’d forgotten to call it.

(Side note: I chose to throttle vs debounce because I didn’t want to wait until someone stopped scrolling and restarted before calling it again, but just ensure that I wasn’t calling it too often.1, 2)

Initially, my throttling function looked something like this:

import * as _ from 'lodash'

const throttledScroll = e => {
  const { target } = e
  const scrollTop = target.scrollTop
  const offset = target.offsetHeight
  const scrollHeight = target.scrollHeight
  _.throttle(scrollTop => {
    if (scrollHeight - top - offset === 0) {
      // do something every 500 ms
    } else {
      // or do something else
    }
  }, 500)
}

Adding this function to a DOM element to listen for a Scroll event emissions will not work. Or rather, it won’t work as expected. The throttledScroll function will be called on every scroll event, but the something I’m expecting to happen will never happen.

I was pulling my hair out trying to figure out why for much longer than I care tot admit. In fact, it took stepping away from the computer and grabbing dinner for me to realize what I’d missed — I had defined a function, but I never called it.

Notice that this is actually a function definition — I’m describing to Lodash what I’ll will want to be throttled when it receives a parameter scrollTop.

_.throttle(scrollTop => {
  if (scrollHeight - scrollTop - offset === 0) {
    // do something every 500 ms
  } else {
    // or do something else
  }
}, 500)

I was blinded by my use of shadowing (the re-use of variable names) scrollTop. I had assumed I’d have access to the scrollTop defined above the _.throttle - which of course, was not the case.

Once I realized the problem, I saw two possible solutions:

  1. Use an Immediately Invoked Function Expression (IIFE)
  2. Use a generic function expression

The IIFE Approach

Turning this functional expression into an IIFE is as easy as wrapping it in parenthesis and then invoking it with a trailing set of (). In our case, we put scrollTop in that trailing parenthesis, because that’s the variable we want to call the throttle with.

_.throttle(top => {
  if (scrollHeight - top - offset === 0) {
    // do something every 500 ms
  } else {
    // or do something else
  }
}, 500)(scrollTop)

Note: I changed the signature to top to differentiate our variable scrollTop defined above and the variable that the _.throttle actually needs in its calculation.

The Generic Approach

This is very similar, however, I’m assigning the function to a new variable, this time, simply, throttle. On the next line, once it’s been assigned, I call it by passing in the variable scrollTop.

const throttle = _.throttle(top => {
  if (scrollHeight - top - offset === 0) {
    // do something every 500 ms
  } else {
    // or do something else
  }
}, 500)
throttle(scrollTop)

Conclusion

Since both of these are function expressions, there are no performance benefits either way as far as I know, so it’s a matter of preference. Ultimately, I went with the generic approach because I felt it was more readable.

The key point is that in order for the throttling to actually occur, I needed to invoke the function!

Live and learn. Hopefully this serves as a good reminder for things to check when my operations are behaving as expected.

Footnotes


Stephen Weiss

Thanks for reading! My name's Stephen Weiss. I live in Chicago with my wife, Kate, and dog, Finn.
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