javascript

Chapter 22: The Evolution of JavaScript
the third is 4, and so on. When myNumbers() completes without calling yield (after the final loop
iteration), calling next() throws a StopIteration error. So to output all numbers in the generator, a
while loop is wrapped in a try - catch statement to prevent the error from stopping code execution.
If a generator is no longer needed, it ’ s best to call the close() method. Doing so ensures that the rest of
the original function is executed, including any finally blocks related to try - catch statements.
Generators are useful when a sequence of values need to be produced and each subsequent value is
somehow related to the previous one.
Iterators
An iterator is an object that iterates over a sequence of values and returns them one at a time. When you
use a for loop or a for - in loop, you ’ re typically iterating over values and processing them one at a
time. Iterators provide the ability to do the same without using a loop. JavaScript 1.7 supports iterators
for all types of objects.
To create an iterator for an object, use the Iterator constructor and pass in the object whose values
should be iterated over. The next() method is used to retrieve the next value in the sequence. By
default, this method returns an array whose first item is the index of the value (for arrays) or the name of
the property (for objects) and whose second item is the value. When no further values are available,
calling next() throws a StopIteration error. Here is an example:
var person = {
name: “Nicholas”,
age: 29
};
var iterator = new Iterator(person);
try {
while(true){
let value = iterator.next();
document.write(value.join(“:”) + “ < br / > ”);
}
} catch(ex){
//intentionally blank
}
This code creates an iterator for the person object. The first time next() is called, the array [ “ name ” ,
“ Nicholas “ ] is returned, and the second call returns [ “ age ” , 29] . The output from this code is as
follows:
name:Nicholas
age:29
When an iterator is created for a non - array object, the properties are returned in the same order as they
would be in a for - in loop. This also means that only instance properties are returned and the order
in which the properties are returned varies upon implementation.
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