javascript

Chapter 22: The Evolution of JavaScript
var evens = [i for each (i in numbers) if (i % 2 == 0)];
//multiply every value by 2
var doubled = [i*2 for each (i in numbers)];
//multiply every odd number by 3
var tripledOdds = [i*3 for each (i in numbers) if (i % 2 > 0)];
All of the array comprehensions in this code use i as a variable to iterate over all values in numbers .
Some of them use conditions to filter the results of the array. Essentially, if the condition evaluates to
true, the value is added to the array. The syntax is a little different from traditional JavaScript but is
more succinct than writing your own for loop to accomplish the same task. Firefox (version 2 and later)
is the only browser to implement this feature, and it requires the type attribute of the < script > element
to be “ application/javascript;version=1.7 ” to enable it.
The values portion of an array comprehension can also be a generator or an iterator.
Destructuring Assignments
It ’ s quite common to have a group of values from which you want to extract one or more into individual
variables. Consider the value returned from an iterator ’ s next() method, which is an array containing
the property name and value. In order to store each in its own variable, it would require two statements,
as in this example:
var nextValue = [“color”, “red”];
var name = nextValue[0];
var value = nextValue[1];
A destructuring assignment allows you to assign both array items into variables using a single statement
such as this:
var [name, value] = [“color”, “red”];
alert(name); //”color”
alert(value); //”red”
In traditional JavaScript syntax, an array literal cannot be on the left side of an assignment. Destructuring
assignment introduces this syntax to indicate that the variables contained in the array to the left of the
equal sign should be assigned the values contained in the array to the right of the equal sign. The result
is that name is filled with “ color ” , and value is filled with “ red ” .
If you don ’ t want all of the values, you can provide variables just for the ones you want, as in this
example:
var [, value] = [“color”, “red”];
alert(value); //”red”
Here, only the variable value is assigned, and it receives the value “ red ” .
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