javascript

Chapter 22: The Evolution of JavaScript
Of these methods, the two most similar are every() and some() , which each query the array for items
matching some criteria. For every() , the passed - in function must return true for every item in order
for the method to return true ; otherwise, it returns false . The some() method, on the other hand,
returns true if even one of the items causes the passed - in function to return true . Here is an example:
var numbers = [1,2,3,4,5,4,3,2,1];
var everyResult = numbers.every(function(item, index, array){
return (item > 2);
});
alert(everyResult);
//false
var someResult = numbers.some(function(item, index, array){
return (item > 2);
});
alert(someResult);
//true
This code calls both every() and some() with a function that returns true if the given item is greater
than 2. For every() , the result is false because only some of the items fit the criteria. For some() , the
result is true because at least one of the items is greater than 2.
The next method is filter() , which uses the given function to determine if an item should be included
in the array that it returns. For example, to return an array of all numbers greater than 2, the following
code can be used:
var numbers = [1,2,3,4,5,4,3,2,1];
var filterResult = numbers.filter(function(item, index, array){
return (item > 2);
});
alert(filterResult); //[3,4,5,4,3]
Here, an array containing the values 3, 4, 5, 4, and 3 is created and returned by the call to
filter() because the passed - in function returns true for each of those items. This method is very
helpful when querying an array for all items matching some criteria.
The map() method also returns an array, but the contents of the array are equal to the return value of the
passed - in function for each item. For example, you can multiply every number in an array by two and
are returned an array of those numbers, as shown here:
var numbers = [1,2,3,4,5,4,3,2,1];
var mapResult = numbers.map(function(item, index, array){
return item * 2;
});
alert(mapResult); //[2,4,6,8,10,8,6,4,2]
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