JavaScript Promise

A Promise in JavaScript represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a cleaner alternative to deeply nested callback functions, making asynchronous code more readable and manageable.

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What Is a Promise?

A Promise is an object that may be in one of three states:

• Pending – Initial state, neither fulfilled nor rejected.
• Fulfilled – Operation completed successfully.
• Rejected – Operation failed with an error.

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Syntax

• resolve(value) – called if the operation is successful.
• reject(error) – called if the operation fails.

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Example 1: Basic Promise

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Example 2: Simulating Asynchronous API with setTimeout

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Chaining Promises

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Handling Errors with catch()

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Promise with finally()

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Key Takeaways

• Promises handle asynchronous operations more cleanly than callbacks.
• A Promise has three states: pending, fulfilled, and rejected.
• Use .then() for success, .catch() for errors, and .finally() to run code regardless of outcome.
• Promises can be chained to run multiple sequential async operations.