Why does a parent element react when I click a nested button?
Learn why a click on a nested button can run a parent handler, and use target and currentTarget to identify what happened.
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How can I select one specific element without accidentally matching several?
Choose a selector based on whether you know a unique ID, need a CSS match, or want every match—and check the result before updating it.
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What is the difference between global, function, and block scope?
Learn where JavaScript variables declared with `var`, `let`, and `const` can be used, with examples you can check yourself.
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How are `const` objects different from deeply immutable objects?
Understand what `const` protects, which object changes remain allowed, and how to show reassignment as an invalid example.
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When does JavaScript use the `else` branch of an `if` statement?
Learn when JavaScript chooses an `else` branch, why zero takes it, and how two falsy inputs behave in a small example.
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What is the difference between a function declaration and a function expression?
Compare two JavaScript function forms, see each return the same result, and use a checklist to identify their syntax and calls.
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When should I choose `for...of` instead of `for...in`?
See what each loop produces on an array, when to choose it, and a quick check to avoid looping over the wrong thing.
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How can I safely read a nested property that may be missing?
Use optional chaining to avoid an access error when a link in a JavaScript property chain is nullish, then decide how your code should handle the missing value.
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When should I use `map`, `filter`, or `reduce` on an array?
Choose by the result you need: transformed items, selected items, or one accumulated result. See all three with the same small array.
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How do `||` and `??` differ when choosing a fallback value?
Compare how `||` and `??` handle `0`, empty strings, and `null`, then choose the fallback behavior that matches your data.
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What does truthy or falsy mean in JavaScript?
Learn how JavaScript treats values in conditions, which values are falsy, and why empty arrays and objects still pass an if check.
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What is the difference between `==` and `===` in JavaScript?
Compare JavaScript’s loose and strict equality operators with examples, then use a practical rule and checklist to choose between them.
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How do JavaScript's primitive values differ from objects?
Learn the seven primitive types, how object identity differs, and how to use `typeof` without being misled by `null`.
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How can I tell the difference between `undefined` and `null` in JavaScript?
Learn what `undefined` and `null` commonly indicate, how strict equality distinguishes them, and how to choose a check that fits your code.
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How can CSS honor a user’s reduced-motion preference for transitions?
Use prefers-reduced-motion to remove or simplify nonessential transitions, while keeping state changes clear and checking both preference settings.
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Why does my CSS transition not run when I change a property?
Check whether the value changes, the transition covers that property, a nonzero duration applies, and no later rule overrides it.
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Why does opacity make a whole element translucent instead of just its background?
Understand why text fades with an element’s opacity, and compare it with a background color that has its own transparency.
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When should I use px, em, or rem for CSS values?
Compare px, em, and rem for typography and spacing, with a worked example showing how nested em font sizes can compound.
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Why does a CSS declaration appear in the stylesheet but not take effect?
Trace a visible CSS declaration through syntax, browser support, selector matching, conditions, and the cascade to find why it has no effect.
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Why does a CSS property inherit on one element but not another?
Learn why a child gets some parent styles but not others, and use a small CSS example and checklist to trace the computed value.
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Why does a later CSS rule lose even though it appears last?
Use the browser’s Styles panel and a cascade checklist to find why a later CSS declaration loses, then choose a focused fix.
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How do :is(), :where(), and :not() affect selector specificity?
Compare how the three selector functions contribute specificity, then use worked examples and a checklist to choose rules that are easier to override.
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How is specificity calculated when several CSS rules match one element?
Learn to compare matching selectors by ID, class-like, and type columns, then use a worked example to identify the winning declaration.
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Why does a CSS selector match more elements than I expected?
Trace a selector against a small HTML tree to see how spaces, child and sibling combinators, and comma-separated branches affect its matches.
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