JavaScript Generators Quiz

Midterm Review

• Score Distribution:

  • 95% High Score
  • 33% Low Score
  • 71% Mean Score
  • 70% Median Score
  • 00:52:42 Mean Elapsed Time

• Student Scores:

  • Approximately 9 students scored in the 0% - 10% range
  • Approximately 20 students scored in the 60% - 70% range
  • Approximately 18 students scored in the 70% - 80% range
  • Approximately 15 students scored in the 80% - 90% range
  • Approximately 9 students scored in the 90% - 100% range

Generators in JavaScript

• Generators:
  • Introduced in ES6
  • Allow a function to generate a sequence of values over time.
  • Pause and resume execution instead of computing everything at once.
  • Useful for iterators and asynchronous programming.
  • "Remember" the state of the function when it pauses.
• Generator Function Syntax:
  • function* simpleGenerator() { ... } (note the asterisk)

Yield Keyword

• Yield:
  • Pauses execution at that point and resumes on a subsequent call to the generator.
  • Can return a value to the caller.
• Function Completion:
  • When the end of a generator function is reached, it returns 'undefined'.
  • Cannot be resumed after function completion.
• Yield Restrictions:
  • yield can only be used within a generator function.

Value Returned by Yield

• IteratorResult Object:
  • Yield returns an IteratorResult object.
  • Has value and done properties.
  • value: The value returned by the yield statement.
  • done: Indicates whether the iteration is finished (true) or not (false).
• Caller Responsibility:
  • The caller needs to check the done property to know when the iteration is complete.
• Exceptions and Returns:
  • A generator can throw an exception, which can be caught.
  • Returning a value sets done to true and the value in IteratorResult.

SimpleGenerator

• Example Code:
  • function* simpleGenerator() { yield 1; yield 2; yield 3; }
  • const g = simpleGenerator();
  • console.log(g.next()); // Output: {value: 1, done: false}
  • console.log(g.next()); // Output: {value: 2, done: false}
  • console.log(g.next()); // Output: {value: 3, done: false}
  • console.log(g.next()); // Output: {value: undefined, done: true}

Use Cases (Iteration)

• generateID() Generator function:
  • Creates an infinite sequence of incrementing IDs.
  • Demonstrates generator use for iterating over sequences.

Class Activity - 1

• Factors Generator Function:
  • Write a generator function to yield the factors of a natural number n.
  • Calculates factors on demand, not ahead of time.

Exercise - Iterating over a list of names

• Name Iterator Generator:
  • Create a generator function to iterate over a list of names.
  • Yield each name one at a time.

Exercise - Word Occurrences

• findWordOccurrences Generator:
  • Iterate over a large string to find occurrences of a given word.
  • Yields the number of characters traversed after the previous match.
• countWordOccurrences Function:
  • Iterate over the findWordOccurrences generator and count the total occurrences of a word.

Generators and Asynchronous Callbacks

• Generator Limitations:
  • Generators cannot easily call asynchronous functions.
  • Callbacks within generators cannot directly use yield.
• Promise Solution:
  • Use promises to manage asynchronous operations.

Generators and Promises

• Yield Promises:
  • Yield promises instead of values within the generator function.
  • Promises are handled like regular synchronous yields.
• Attach .then and .catch:
  • Attach .then and .catch to the yielded promises to handle resolutions and rejections, respectively.

Exercise - Asynchronous Function Execution

• asyncFunctionExecutor Generator:
  • Executes functions in an array asynchronously with a specified delay between executions.
  • Passes the current index to each function as an argument.