Piotr Kulpinski

Supercharge your number inputs with custom step sizes

Apr 26, 2024

#engineering

The <input type="number"> element provides a convenient way to handle numeric input in web forms. You can set bounds with the min and max attributes, and users can press the up and down arrow keys to increment or decrement the value by the specified step size. However, what if you want to allow users to adjust the value using different step sizes?

By default, the step attribute not only determines the increment/decrement amount but also clips the value to the nearest multiple of the step. For example, if you have an input with a value of 5 and a step of 10, pressing the up arrow key will set the value to 10 (the nearest multiple of the step) instead of 15 (5 + 10).

How to build a better number input

In this article, we’ll explore how to enhance the functionality of number inputs to support custom step sizes. We’ll implement the following features:

Here’s a preview of what we’ll be building:

Let’s dive into the implementation!

Helper Functions

First, let’s define some utility functions that we’ll use throughout the implementation.

const parseNumericValue = (value?: string | number | null) => {
  if (value === undefined || value === null) return undefined
  const parsed = Number.parseFloat(value.toString())

  return Number.isNaN(parsed) ? undefined : parsed
}

The parseNumericValue function takes a value (which can be a string, number, or null) and parses it as a floating-point number. If the value is undefined or null, it returns undefined. If the parsing fails, it also returns undefined.

This function will be useful for parsing the min, max, step, and current value of the input elements.

const preciseRound = (value: number, decimals = 2) => {
  const factor = Math.pow(10, decimals)

  return Math.round((value + Number.EPSILON) * factor) / factor
}

The preciseRound function rounds a number to a specified number of decimal places. It uses the Number.EPSILON constant to handle floating-point precision issues.

const keepNumberInRange = (value: number, min?: number, max?: number) => {
  if (min !== undefined && max !== undefined) {
    return Math.min(Math.max(value, min), max)
  }
  if (min !== undefined) {
    return Math.max(value, min)
  }
  if (max !== undefined) {
    return Math.min(value, max)
  }

  return value
}

The keepNumberInRange function ensures that a given number stays within the specified min and max bounds. I like to use reusable utility functions like this to keep the code clean and maintainable.

Implementing the Key Up Handler

Here’s the main event handler function that will be called when the user presses the up or down arrow keys on a number input:

import { KeyboardEvent } from "react"

const handleKeyUp = (e: KeyboardEvent<HTMLInputElement>) => {
  if (!["ArrowUp", "ArrowDown"].includes(e.key)) {
    return
  }

  const target = e.currentTarget
  const direction = e.key === "ArrowUp" ? 1 : -1
  const min = parseNumericValue(target.getAttribute("min"))
  const max = parseNumericValue(target.getAttribute("max"))
  const step = parseNumericValue(target.getAttribute("step")) || 1
  const value = parseNumericValue(target.value) || min || 0
  const increment = step * (e.metaKey ? 100 : e.shiftKey ? 10 : e.altKey ? 1 / 10 : 1)

  const newValue = preciseRound(value + direction * Math.max(increment, 0.1), 2)

  if (newValue !== value) {
    target.value = keepNumberInRange(newValue, min, max)
    e.preventDefault()
  }
}

// Usage: <input type="number" onKeyUp={handleKeyUp} />

Let’s break down the code:

  1. We check if the pressed key is either “ArrowUp” or “ArrowDown”. If not, we return early.
  2. We get the current target input element and determine the direction based on the pressed key (+1 for “ArrowUp”, -1 for “ArrowDown”).
  3. We parse the min, max, and step attributes of the input element using the parseNumericValue utility function.
  4. We parse the current value of the input using parseNumericValue or fallback to the min value or 0 if the input is empty.
  5. We calculate the increment based on the modifier keys: Ctrl/Cmd multiplies the step by 100, Shift multiplies it by 10, Alt divides it by 10, and the default is 1.
  6. We calculate the new value by adding the product of the direction and the increment to the current value, using the preciseRound utility function to handle floating-point precision.
  7. If the new value is different from the current value, we update the input value using the setInputValue function, ensuring it stays within the specified min and max range.
  8. Finally, we call e.preventDefault() to prevent the default browser behavior.

With this implementation, your number inputs will have the desired supercharged behavior, allowing users to quickly adjust values using different step sizes based on the modifier keys they press.

You can easily integrate this code into your project by attaching the handleKeyUp event handler to your number input elements.

Happy coding!

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