When working with JavaScript to manipulate the style properties of HTML elements, one might wonder why a line of code like this appears to work without explicitly specifying units:
playArea.game.style.width = cols * 100 + (cols * 2);
At first glance, this line might seem incomplete because, in CSS, units such as px, %, em, or rem are required to define the width of an element. Without a unit, it’s not immediately clear how the browser interprets this value. Let’s break this down and explore why this happens, and what the correct approach should be.
Understanding CSS Style Properties in JavaScript
In CSS, when setting the width property of an element, the browser expects a specific format. This format consists of a numeric value followed by a unit, like:
width: 200px;
However, when using JavaScript to manipulate styles, you’re interacting with the DOM (Document Object Model) directly through JavaScript objects, and JavaScript operates differently from CSS. The value you assign to a style property like width is converted to a string by JavaScript.
Why the Line Without Units “Works”
In your example:
playArea.game.style.width = cols * 100 + (cols * 2);
This line assigns a number to style.width, but that number is treated as a string. Since you didn’t specify units, JavaScript interprets it as a number and passes it along to the browser’s layout engine.
Browsers are lenient in handling some cases where CSS units are omitted, especially when dealing with certain properties like width, height, and font-size. When no unit is specified, some browsers may still apply the value, interpreting it as a pixel (px) value by default. But this behavior is inconsistent across different browsers, and modern standards expect you to provide explicit units.
Therefore, while this line might work in some environments, it’s not guaranteed to work universally.
The Correct Approach: Adding Units
In JavaScript, when you assign a width (or any style property that requires units) to an element, you should always provide the proper unit. Otherwise, you may encounter issues where your code works in one browser but fails in another, or it might stop working if the browser updates its behavior.
Here’s the corrected version:
playArea.game.style.width = `${cols * 100 + (cols * 2)}px`;
Using template literals (the ${} syntax) ensures that the value is concatenated with the px unit, creating a string that JavaScript and the browser will both interpret correctly as a valid width.
Why the Correct Version Works
By using the template literal, you are explicitly specifying the unit:
`${cols * 100 + (cols * 2)}px`
This converts the entire expression cols * 100 + (cols * 2) into a string with the px suffix. Now, when you assign this string to style.width, the browser knows to apply the width in pixels. Without specifying px, the browser might not understand what unit to use, leading to unpredictable results.
Common Mistake: Missing Units in JavaScript CSS Manipulation
Many developers overlook the need to specify units when working with style properties in JavaScript. The result can be inconsistent behavior across browsers, as some might interpret the lack of units differently or ignore the value altogether.
It’s always a good practice to:
- Calculate the numeric value as needed.
- Concatenate the value with the appropriate unit (e.g.,
px,%,em, etc.). - Ensure that the final value is passed as a string with both the number and the unit.
Key Takeaway
Even though the line playArea.game.style.width = cols * 100 + (cols * 2) may seem to work in some cases, it’s relying on browser quirks. The correct approach is to always include units, ensuring compatibility and predictable behavior across different browsers and environments. The best practice is to write:
playArea.game.style.width = `${cols * 100 + (cols * 2)}px`;
By explicitly specifying the unit, you’ll avoid potential issues and ensure your code works consistently across all platforms.
