What does the "yield" keyword do in Python and Javascript?
Tags: javascript, python, programming, beginners
Published: 2023-12-12 20:27:35

The "yield" keyword in both Python and Javascript plays a key role in working with generators, but with some subtle differences:

## Python:

Function creation: yield is used to create generator functions, which are functions that can pause and resume their execution multiple times, yielding values each time they resume. This allows for memory-efficient iteration over large datasets or lazy evaluation of expressions.
Pausing and resuming: Within a generator function, yield pauses execution and returns a value to the caller. The function can then resume execution when the caller requests the next value using an iterator interface.

```
def fibonacci(n):
  a, b = 0, 1
  for _ in range(n):
    yield a
    a, b = b, a + b

# Use the generator with a for loop
for num in fibonacci(10):
  print(num)
```

This code generates the first 10 Fibonacci numbers without storing them all in memory at once.

## Javascript:

Async iteration: yield is used with the async and await keywords to pause and resume execution in asynchronous code. It allows for iterating over asynchronous operations one by one, similar to a regular loop.
Returning values: Similar to Python, yield within an async generator function returns a value to the caller. However, it doesn't pause execution; instead, it signals that the next value is ready when the caller resumes.

```
async function fetchUserRepos(username) {
  const response = await fetch(`https://api.github.com/users/${username}/repos`);
  const repos = await response.json();

  for (const repo of repos) {
    yield repo.name;
  }
}

(async () => {
  const repoNames = [];
  for await (const name of fetchUserRepos('<insert_userName_here>')) {
    repoNames.push(name);
  }
  console.log(repoNames); // Prints all repository names
})();
```

This code fetches a user's repositories from Github and iterates over them one by one, waiting for each repository to be fetched before proceeding.

## Key differences:


<table>
  <thead>
    <tr>
      <th>Feature</th>
      <th>Python</th>
      <th>Javascript</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Execution</td>
      <td>Synchronous</td>
      <td>Asynchronous</td>
    </tr>
    <tr>
      <td>Value consumption</td>
      <td><code>for</code> loop or iterators</td>
      <td><code>next()</code> method</td>
    </tr>
    <tr>
      <td>Generator completion</td>
      <td><code>return</code> statement or end of function</td>
      <td><code>return</code> statement or end of function</td>
    </tr>
  </tbody>
</table>


## Purpose:
 In Python, yield is primarily used for memory-efficient iteration and lazy evaluation. In Javascript, it's used for asynchronous iteration and managing control flow in async functions.

## Execution:
 Python's yield pauses execution completely, while Javascript's yield only signals that a value is ready for the caller.

I hope this clarifies the role of yield in both Python and Javascript. Feel free to ask if you have further questions!

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