CODEWITHSUNDEEP
javascriptarrays
pierrotlefou
pierrotlefou

Reputation: 40721

array.select() in JavaScript

Does JavaScript has similar functionality as Ruby has?

array.select {|x| x > 3}

Something like:

array.select(function(x) { if (x > 3)  return true})

Upvotes: 108

Views: 269974

Answers (6)

Bamdad
Bamdad

Reputation: 838

In some cases, you can also use the map function as shown below:

var celsiusValues = [22, 50, 310, 0, 80];
var kelvinValues = celsiusValues.map(function(x) { return x - 273; });
console.log(kelvinValues); //[-251, -223, 37, -273, -193]

or 

var people = [ {name: 'Mary', gender: 0, age: 28 }, {name: 'Sara', gender: 0, age: 17 }, {name: 'Tom', gender: 1, age: 20 }];
var titles = people.map(function(x) { return (x.gender == 0? 'Mrs.' : 'Mr.') + x.name; });
console.log(titles); //["Mrs.Mary", "Mrs.Sara", "Mr.Tom"]

var ageVrification = people.map(function(x) { return x.age >= 18; });
console.log(ageVrification); //[true, false, true]

Upvotes: 4

BoltClock
BoltClock

Reputation: 723618

There is Array.filter():

var numbers = [1, 2, 3, 4, 5];
var filtered = numbers.filter(function(x) { return x > 3; });

// As a JavaScript 1.8 expression closure
filtered = numbers.filter(function(x) x > 3);

Note that Array.filter() is not standard ECMAScript, and it does not appear in ECMAScript specs older than ES5 (thanks Yi Jiang and jAndy). As such, it may not be supported by other ECMAScript dialects like JScript (on MSIE).

Nov 2020 Update: Array.filter is now supported across all major browsers.

Upvotes: 167

Starcat
Starcat

Reputation: 691

There's also Array.find() in ES6 which returns the first matching element it finds.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find

const myArray = [1, 2, 3]

const myElement = myArray.find((element) => element === 2)

console.log(myElement)
// => 2

Upvotes: 20

xinghua
xinghua

Reputation: 149

Array.filter is not implemented in many browsers,It is better to define this function if it does not exist.

The source code for Array.prototype is posted in MDN

if (!Array.prototype.filter)
{
  Array.prototype.filter = function(fun /*, thisp */)
  {
    "use strict";

    if (this == null)
      throw new TypeError();

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun != "function")
      throw new TypeError();

    var res = [];
    var thisp = arguments[1];
    for (var i = 0; i < len; i++)
    {
      if (i in t)
      {
        var val = t[i]; // in case fun mutates this
        if (fun.call(thisp, val, i, t))
          res.push(val);
      }
    }

    return res;
  };
}

see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter for more details

Upvotes: 4

silly
silly

Reputation: 7887

yo can extend your JS with a select method like this

Array.prototype.select = function(closure){
    for(var n = 0; n < this.length; n++) {
        if(closure(this[n])){
            return this[n];
        }
    }

    return null;
};

now you can use this:

var x = [1,2,3,4];

var a = x.select(function(v) {
    return v == 2;
});

console.log(a);

or for objects in a array

var x = [{id: 1, a: true},
    {id: 2, a: true},
    {id: 3, a: true},
    {id: 4, a: true}];

var a = x.select(function(obj) {
    return obj.id = 2;
});

console.log(a);

Upvotes: 6

Tim
Tim

Reputation: 1006

Underscore.js is a good library for these sorts of operations - it uses the builtin routines such as Array.filter if available, or uses its own if not.

http://documentcloud.github.com/underscore/

The docs will give an idea of use - the javascript lambda syntax is nowhere near as succinct as ruby or others (I always forget to add an explicit return statement for example) and scope is another easy way to get caught out, but you can do most things quite easily with the exception of constructs such as lazy list comprehensions.

From the docs for .select() (.filter() is an alias for the same)

Looks through each value in the list, returning an array of all the values that pass a truth test (iterator). Delegates to the native filter method, if it exists.

  var evens = _.select([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
  => [2, 4, 6]

Upvotes: 6

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