QJavaScript · Interview preparation
Why does changing an array sometimes change another variable?
Assigning an array to another variable copies its reference, not its contents.
The idea to remember.
Assigning an array to another variable copies its reference, not its contents. Create a new array and copy nested mutable values when you need independent state.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: transform a list
01 / BEGINNERMap builds a new array without changing the original.
const prices = [5, 10, 15];
const discounted = prices.map(price => price * 0.9);Intermediate: group with reduce
02 / INTERMEDIATEReduce can create a lookup keyed by a stable ID.
const usersById = users.reduce((map, user) => {
map[user.id] = user;
return map;
}, {});Real scenario: efficient list filtering
03 / REAL SCENARIODerive results without mutating your source collection.
function visibleProducts(products, query, category) {
const term = query.trim().toLowerCase();
return products.filter(item =>
(!category || item.category === category) &&
item.name.toLowerCase().includes(term)
);
}02Check your understanding
Try it in your own words.
Explain why does changing an array sometimes change another variable without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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