JavaScript Generators Quiz

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Questions and Answers

What is the primary function of a generator in JavaScript?

To compute all values at once.

To process synchronous programming efficiently.

To create infinite loops in a function.

To generate a sequence of values over time with pausing capabilities. (correct)

Which syntax is used to define a generator function?

function* name() {} (correct)

generator function name() {}

function name*() {}

function => generator() {}

What does the 'yield' keyword do in a generator function?

Allows calling a nested function within the generator.

Terminates the generator permanently.

Pauses execution at that point while retaining the state. (correct)

Returns all values to the caller simultaneously.

What is returned when a generator's execution reaches the end?

undefined or specified return value Signup and view all the answers

In what form does the yield statement return its value?

As an IteratorResult object. Signup and view all the answers

Which action can be performed by a generator besides yielding values?

Throw exceptions that can be caught via try-catch. Signup and view all the answers

What must be checked by the caller when using a generator?

The value of 'done' property. Signup and view all the answers

How does control flow work with the yield statement?

Control is ceded back to the caller, retaining function state. Signup and view all the answers

What does invoking 'return' in a generator function do?

Sets the value of IteratorResult to the return value and done to true. Signup and view all the answers

What will the output of 'console.log(g.next());' be after the third yield in the simpleGenerator function?

{ value: 3, done: false } Signup and view all the answers

In the fetchData generator function, what does 'yield' before the fetch function call signify?

It waits for the fetch call to complete before proceeding. Signup and view all the answers

How does the generateID function behave when called multiple times with 'g.next()'?

It returns increasing IDs successively starting from 1. Signup and view all the answers

What will be the output of 'console.log(g.next());' after a generator has reached the end of its yields?

{ value: undefined, done: true } Signup and view all the answers

What is the purpose of using a generator to yield the factors of a natural number n?

Yields each factor on demand without pre-calculation. Signup and view all the answers

What will happen if no value is returned from a generator function?

The done property will be true and the value will be undefined. Signup and view all the answers

What is the output of 'console.log(g.next());' when the generator function generates IDs starting from 1?

ID will be 1 on the first call and increment after each next call. Signup and view all the answers

Study Notes

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.

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Description

Test your knowledge on JavaScript generators and the yield keyword introduced in ES6. This quiz covers key concepts, syntax, and use cases of generators in programming. Perfect for those wanting to enhance their understanding of asynchronous programming!