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Object.freeze(): locked container, not locked nested content
Freezing is not automatically recursive.
Published22 Oct 2026
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Object.fromEntries(): back from pairs to object
Object.fromEntries() creates an object of key-value pairs. Check the repeated keys if you don't want later values to replace earlier ones.
Published22 Oct 2026
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Object.hasOwn(): existing field with undefined
The presence of a property is different from the presence of a useful value.
Published22 Oct 2026
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Object.is(): special cases NaN and negative zero
The comparison has small but important differences compared with ===.
Published22 Oct 2026
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Object.keys(): names of own enumerable properties
Inherited properties do not automatically enter the list.
Published22 Oct 2026
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Object.values(): values without their names
When converting, the direct connection to the keys in the received array is lost.
Published22 Oct 2026
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Opacity: transparent does not mean inactive
opacity affects the entire element together with its contents. At opacity: 0, it can remain in layout, receive pointer events and participate in keyboard focus.
Published23 Oct 2026
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Optional chaining: access to a property that may be missing
The ?. operator terminates access at null or undefined. It does not hide arbitrary errors in performance.
Published22 Oct 2026
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Order: visual rearrangement does not rewrite the document
order changes visual arrangement in flex and grid containers. Items are sorted by value, with source order resolving ties.
Published23 Oct 2026
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Os.environ: settings from the process environment
os.environ exposes process environment variables as text. A configured port, for example, needs conversion and validation before being used as a number.
Published22 Oct 2026
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outline and border: the difference in box size
The contour can highlight an element without changing the arrangement.
Published22 Oct 2026
Technology
Outline-color: focus must remain visible on the actual background
outline-color sets the outline's colour. When customising keyboard focus, it needs to remain distinguishable from the surrounding background.
Published23 Oct 2026