How to get a key in a JavaScript object by its value?

Dealing with non-unique values

Imagine you're dealing with non-unique values and you're tasked to fetch all the keys that share this common value. You can use filter instead of find like this:

const keys = Object.keys(obj).filter(k => obj[k] === value); // Collects all the keys, no keys will feel left out!
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This will cough up an array full of keys that share the same value.

Encapsulating in a function

To prevent yourself from repeating this logic, encapsulate it in a function. This way you can keep your code DRY (Don't Repeat Yourself) and your sanity intact.

function getKeyByValue(object, value) {
    return Object.keys(object).find(key => object[key] === value); // Reusable magic, not just a one-trick pony!
}
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Congratulations, you've just got yourself a new fancy tool in your developer's toolkit.

Going big with large objects

When working with larger objects or if you're wary about performance, consider using a "hashmap". In essence, it's creating a value-to-key map for swift retrieval:

const valueKeyMap = new Map(Object.entries(obj).map(([key, value]) => [value, key])); // Map O'ver speed!
const key = valueKeyMap.get(value);
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This minor setup tweaks the constant O(1) retrieval time, thus ensuring efficiency supremacy.

Wrestling with nested objects and arrays

If your object has nested objects or arrays, JSON.stringify can be your wingman to track down the elusive key:

const keyNested = Object.keys(obj).find(k => JSON.stringify(obj[k]) === JSON.stringify(value)); // When keys play hide and seek!
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This tactic comes with a caution sign due to potential performance hiccups along with quirks around object ordering and inconsistent data types.

Harmony with utility libraries

If your project is already hitched with utility libraries such as Underscore.js or Lodash, you can utilize their innate methods:

// With Underscore.js
const key = _.findKey(obj, val => val === value); // Underscore.js to the rescue!
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These libraries boast about their optimized algorithms and provide the bonus of additional error handling mechanics.

A laundry list of caveats

Always expect edge cases:

• Type coercion: Assure the value types are similar or else resort to == for loose comparison.
• Existential crisis of properties: Reconfirm key existence to steer clear of uncalled errors.
• The unseen property: Object.keys only returns the visible properties, hence Invoke Object.getOwnPropertyNames for an all-key inclusive search.

A tidbit on prescribed practices

Few best practices to adhere to:

• Avoid playing doctor Frankenstein with native object prototypes to prevent mutation mayhem and unpredictable results.
• Simplicity is the ultimate sophistication: Make sure your code is easy to read and easy to debug.
• Be a trendsetter: Don't shy away from ES6 features for a more concise and expressive codebase.