问题
I'm learning JavaScript using W3C and I didn't find an answer to this question.
I'm trying to make some manipulations on array elements which fulfill some condition.
Is there a way to do it other than running on the array elements in for loop? Maybe something like (in other languages):
foreach (object t in tArray)
if (t follows some condition...) t++;
another thing, sometimes I want to use the element's value and sometimes I want to use it as a reference. what is the syntactical difference?
As well, I'll be happy for recommendations on more extensive sites to learn JavaScript from. thanks
回答1:
In most browsers (not IE <= 8) arrays have a filter method, which doesn't do quite what you want but does create you an array of elements of the original array that satisfy a certain condition:
function isGreaterThanFive(x) {
return x > 5;
}
[1, 10, 4, 6].filter(isGreaterThanFive); // Returns [10, 6]
Mozilla Developer Network has a lot of good JavaScript resources.
回答2:
Use ES6 Array.filter() and arrow functions with expression body:
myArray.filter(x => x > 5)
A bit more concise than @Beauty's answer.
回答3:
Here a short way to write a filter. From an array of numbers it returns all values greater than 5.
myArray.filter((x) => { return x > 5; })
Usage example:
var filterResult = [1, 10, 4, 6].filter((x) => { return x > 5; });
console.log(filterResult); // returns [ 10, 6 ]
And here a filter for an array of objects, which checks a property condition.
myArray.filter((x) => { return x.myNumber > 5; })
Usage example:
var myArray = [{myNumber: 1, name: 'one'}, {myNumber: 3, name: 'tree'}, {myNumber: 6, name: 'six'}, {myNumber: 8, name: 'eight'}];
var result = myArray.filter((x) => { return x.myNumber > 5; });
console.log(result); // returns [ { myNumber: 6, name: 'six' }, { myNumber: 8, name: 'eight' } ]
回答4:
You can use for ... in in JavaScript:
for (var key in array) {
if (/* some condition */) {
// ...
}
}
As of JavaScript 1.6, you can use this, too:
for each (var element in array) {
// ...
}
These are mainly meant to traverse object properties. You should consider to simply use your for-loop.
EDIT: You could use a JavaScript framework like jQuery to eliminate these cross-browser problems. Give it a try. Its $.each()-method does the job.
回答5:
About arrays
What you usually want for iterating over array is the forEach method:
arr.forEach(function(el) {
alert(el);
});
In your specific case for incrementing each element of array, I'd recommend the map method:
arr = arr.map(function(t){ return t+1; });
There are also filter, reduce, and others, which too come in handy.
But like Tim Down already mentioned, these won't work by default in IE. But you can easily add these methods for IE too, like shown in MDC documentation, or actually you can even write simpler versions than the ones in MDC documentation (I don't know why they are so un-JavaScript-y over there):
if (!Array.prototype.forEach) {
Array.prototype.forEach = function(func, scope) {
for (var i = 0, len = this.length; i < len; i++) {
func.call(scope, this[i], i, this);
}
};
}
But don't use the for ... in construct for arrays - this is meant for objects.
About references
Another thing, sometimes I want to use the element's value and sometimes I want to use it as a reference. What is the syntactical difference?
In JavaScript every variable is in fact a reference to some object. But those references are passed around by value. Let me explain...
You can pass an object to a function that modifies the object and the changes will be seen outside the function:
function incrementHeight(person) {
person.height = person.height + 1;
}
var p = {height: 10);
alert(p.height); // outputs: 10
incrementHeight(p);
alert(p.height); // outputs: 11
Here you modify the value to which the person reference points to and so the change will be reflected outside the function.
But something like this fails:
function incrementHeight(height) {
height = height + 1;
}
var h = 10;
alert(h); // outputs: 10
incrementHeight(h);
alert(h); // outputs: 10
Here you create a completely new object 11 and assign its reference to variable height. But variable h outside the function still contains the old reference and so remains to point at 10.
回答6:
You can use Array.prototype.find, wich does exactly what you want, returns the first element fullfilling the condition. Example:
> ([4, {a:7}, 7, {a:5, k:'r'}, 8]).find(o => o.a == 5)
{a:5, k:'r'}
回答7:
Just came across the same problem. Tim Down came close, he just needed a wrapper to the length of the filtered array:
// count elements fulfilling a condition
Array.prototype.count = function (f) {
return this.filter(f).length;
};
Usage:
// get the answer weight from the question's values array
var w = Math.pow(q.values.count(function(v) { return v !== -1; }), -1);
I hope that answers this long standing question!
回答8:
Write a generic function that accepts various conditions:
function array_only(arr, condition) {
hold_test=[]
arr.map(function(e, i) {if(eval(condition)){hold_test.push(e)}})
return(hold_test)
}
Example:
use_array = ['hello', 'go_there', 'now', 'go_here', 'hello.png', 'gogo.log', 'hoho.png']
Usage:
return only elements containing .log extension:
array_only(use_array, "e.includes('.log')")
[ 'gogo.log' ]
return only elements containing .png extension:
array_only(use_array, "e.includes('.png')")
[ 'hello.png', 'hoho.png' ]
return only elements NOT containing .png extension:
array_only(use_array, "!e.includes('.png')")
[ 'hello', 'go_there', 'now', 'go_here', 'gogo.log' ]
return elements containing set of extensions and prefixes:
array_only(use_array, "['go_', '.png', '.log'].some(el => e.includes(el))")
[ 'go_there', 'go_here', 'hello.png', 'gogo.log', 'hoho.png' ]
You can easily pass MULTIPLE CONDITIONS
return all png files that are less than 9 characters long:
array_only(use_array, "e.includes('.png') && e.length<9")
[ 'hoho.png' ]
来源：https://stackoverflow.com/questions/3777200/javascript-get-array-element-fulfilling-a-condition