javascript

Chapter 22: The Evolution of JavaScript
In this code, the let statement defines an area within which the num variable is equal to 10 and the
multiplier variable is equal to 2. This definition of num overrides the previously declared value using
var , so within the let statement the result of multiplying by the multiplier is 20. Outside the let
statement, the value of num remains 5. Since each let statement creates its own scope, the variable
values inside it have no bearing on the values outside.
You can use a similar syntax to create a let expression where variable values are set only for a single
expression. Here is an example:
var result = let(num=10, multiplier=2) num * multiplier;
alert(result); //20
Here, a let expression is used to calculate a value using two variables. The value is then stored in the
result variable. After that point, the variables num and multiplier no longer exist.
Using block - level scopes in JavaScript gives you more control over which variables exist at what point
during code execution.
Generators
A generator is an object that generates a sequence of values one at a time. With JavaScript 1.7, you can
create a generator by defining a function that returns a specific value using the yield operator. When a
function is called that uses yield , a new Generator instance is created and returned. The next()
method can then be called to retrieve the first value of the generator. When this happens, the original
function is executed and stops execution when it comes to yield , returning the specified value. In this
way, yield works in a similar manner to return . If next() is called again, code execution continues at
the next statement following yield and then continues to run until yield is encountered again, at
which point a new value is returned. Here is an example:
function myNumbers(){
for (var i=0; i < 10; i++){
yield i * 2;
}
}
var generator = myNumbers();
try {
while(true){
document.write(generator.next() + “ < br / > ”);
}
} catch(ex){
//intentionally blank
} finally {
generator.close();
}
When the function myNumbers() is called, a generator is returned. The myNumbers() function itself is
very simple, containing a for loop that yields a value each time through the loop. Each call to next()
causes another trip through the for loop and returns the next value. The first value is 0, the second is 2,
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