JAVA SCRIPT - Creating Functions that Gracefully Yield

 Creating Functions that Gracefully Yield 

Problem

It’s simple to break out of a function with return, but you want to be able to get back into the function at that point in the future, and then have it resume.

Solution 

Use an ES 6 generator function:

function* taskRunner() {
 console.log('doing something');
 yield function () {
console.log('hello from a');
 };
 console.log('doing something after a');
 yield function() {
 console.log('hello from b');
 }
 console.log('doing something after b');
}
var tasks = taskRunner();
tasks.next().value();
tasks.next().value();
tasks.next();
which results in:
doing something
hello from a
doing something after a
hello from b
doing something after b

EXPLAIN

As already noted, it’s easy to break out of a JavaScript function. Just type in a return statement and you’re done. But what if you want to get back into the function at that exact point, and have it continue? Couldn’t be done, or at least not without embedded event handlers and callbacks and such, until ES 6 brought us generators: functions that can be entered and exited and that remember their state.

 The solution demonstrates iterating over a set of tasks, each preceded by a yield state‐ ment with a function expression, though any expression works. Mozilla demonstrates a function generator by providing code for a neverending counter. In this case, the expression is a number incrementation:

function* taskRunner() {
 console.log('doing something');
 yield function () {
console.log('hello from a');
 };
 console.log('doing something after a');
 yield function() {
 console.log('hello from b');
 }
 console.log('doing something after b');
}
var tasks = taskRunner();
tasks.next().value();
tasks.next().value();
tasks.next();

which results in:

doing something
hello from a
doing something after a
hello from b
doing something after b

EXPLAIN

 As already noted, it’s easy to break out of a JavaScript function. Just type in a return statement and you’re done. But what if you want to get back into the function at that exact point, and have it continue? Couldn’t be done, or at least not without embedded event handlers and callbacks and such, until ES 6 brought us generators: functions that can be entered and exited and that remember their state.

The solution demonstrates iterating over a set of tasks, each preceded by a yield state‐ ment with a function expression, though any expression works. Mozilla demonstrates a function generator by providing code for a neverending counter. In this case, the expression is a number incrementation:

function* idMaker(){
 var index = 0;
 while(true)
 yield index++;
}
var gen = idMaker();
console.log(gen.next().value); // 0
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2