Uncaught TypeError: Cannot set property style of #<HTMLElement> which has only a getter

Question

The following code fails in Chrome, Safari, works fine in Firefox

"use strict";
document.body.style = "background-color: green;";

<p>background should be green</p>

Remove the "using strict" and it works.

Is that a bug in Chrome and Safari or a bug in Firefox? MDN says setting the style is valid.

Answer 1

Problem

Not all browsers support assigning assigning a string which contains a textual representation of a CSS declaration block to the style property.

element.style = styleString; // Might not work

Workaround

As a workaround, you can set it as a content attribute, or to the cssText property:

element.setAttribute('style', styleString);

element.style.cssText = styleString;

Standard behavior

On older browsers compliant with DOM L2 Style and ES5, the assignment should

• Throw in strict mode
• Be ignored in non-strict mode.

On newer browsers compliant with CSSOM and ES5, the assignment should

• Always work

Full details

According to the DOM Level 2 Style spec, the style property is defined in the ElementCSSInlineStyle interface as follows:

interface ElementCSSInlineStyle {
  readonly attribute CSSStyleDeclaration  style;
};

Therefore, the style property should be implemented as an accessor property with a getter but without a setter.

Object.getOwnPropertyDescriptor(HTMLElement.prototype, 'style'); /* {
  configurable: true,
  enumerable: true,
  get: function(){...},
  set: undefined
} */

According to ECMAScript 5, when you attempt to assign some value to a property like that, an error must be thrown in strict mode:

When an assignment occurs within strict mode code, [...] the LeftHandSide also may not be a reference [...] to an accessor property with the attribute value {[[Set]]:undefined} [...]. In these cases a TypeError exception is thrown.

However, DOM L2 Style is superseded by the newer CSS Object Model (CSSOM).

According to the that spec, the style IDL attribute of the interface ElementCSSInlineStyle, implemented by HTMLElement, is defined as a [PutForwards] extended attribute:

[NoInterfaceObject]
interface ElementCSSInlineStyle {
  [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
};

That means that setting the style property must behave like setting the cssText one of the CSSStyleDeclaration. Therefore, those must be equivalent:

element.style = styleString;
element.style.cssText = styleString;

And that's why it works on newer browsers.