JavaScript variable assignments from tuples

Question

In other languages like Python 2 and Python 3  you can define and assign values to a tuple variable  and retrieve their values like this   tuple     Bob   24  name  age   tuple print name             name evaluates to Bob print age              age evaluates to 24   Is there anything similar in JavaScript  Or do I just have to do it the ugly way with an array   tuple     Bob   24  name   tuple 0          name Evaluates to Bob age   tuple 1           age Evaluates to 24   Is there a better way to simulate Python tuples in JavaScript 5   Update  See the answer regarding ES6  which should be favored over CoffeeScript for new projects

User · Answer

You can have a tuple type in Javascript as well. Just define it with higher order functions (the academic term is Church encoding):

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const Tuple = (...args) => {_x000D_
  const Tuple = f => f(...args);_x000D_
  return Object.freeze(Object.assign(Tuple, args));_x000D_
};_x000D_
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const get1 = tx => tx((x, y) => x);_x000D_
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const get2 = tx => tx((x, y) => y);_x000D_
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const bimap = f => g => tx => tx((x, y) => Tuple(f(x), g(y)));_x000D_
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const toArray = tx => tx((...args) => args);_x000D_
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// aux functions_x000D_
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const inc = x => x + 1;_x000D_
const toUpperCase = x => x.toUpperCase();_x000D_
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// mock data_x000D_
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const pair = Tuple(1, "a");_x000D_
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// application_x000D_
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console.assert(get1(pair) === 1);_x000D_
console.assert(get2(pair) === "a");_x000D_
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const {0:x, 1:y} = pair;_x000D_
console.log(x, y); // 1 a_x000D_
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console.log(toArray(bimap(inc) (toUpperCase) (pair))); // [2, "A"]_x000D_
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const map = new Map([Tuple(1, "a"), Tuple(2, "b")]);_x000D_
console.log(map.get(1), map.get(2)); // a b

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Please note that Tuple isn't used as a normal constructor. The solution doesn't rely on the prototype system at all, but solely on higher order functions.

What are the advantages of tuples over Arrays used like tuples? Church encoded tuples are immutable by design and thus prevent side effects caused by mutations. This helps to build more robust applications. Additionally, it is easier to reason about code that distinguishes between Arrays as a collection type (e.g. [a]) and tuples as related data of various types (e.g. (a, b)).

User · Answer

A frozen array behaves identically to a python tuple   const tuple   Object freeze   Bob   24    let  name  age     tuple console debug name       Bob  console debug age      24   Be fancy and define a class  class Tuple extends Array      constructor    items         super    items        Object freeze this           let tuple   new Tuple  Jim   35   let  name  age    tuple  console debug name      Jim console debug age      35 tuple     Bob   24      no effect  console debug name      Jim console debug age      25   Works today in all the latest browsers

User · Answer

Here is a version of Matthew James Davis s answer with the Python tuple methods added in   x000D   x000D  class Tuple extends Array      constructor    items         super    items        Object freeze this         toArray         return     this         toString         return     super toString              count item        var arr   this toArray        var result   0      for var i   0  i  lt  arr length  i             if arr i      item             result                       return result                    let tuple   new Tuple  Jim   35      let  name age    tuple   console log  tuple   tuple  console log  name   name  console log  age   age  x000D   x000D   x000D

User · Answer

This  tuple  feature it is called destructuring in EcmaScript2015 and is soon to be supported by up to date browsers  For the time being  only Firefox and Chrome support it    But hey  you can use a transpiler   The code would look as nice as python   let tuple     Bob   24  let  name  age    tuple  console log name  console log age

User · Answer

Unfortunately you can t use that tuple assignment syntax in  ECMA Java Script   EDIT  Someone linked to Mozilla JS 1 7 - this wouldn t work cross-browser but if that is not required then there s your answer

User · Answer

This is not intended to be actually used in real life  just an interesting exercise  See Why is using the JavaScript eval function a bad idea  for details   This is the closest you can get without resorting to vendor-specific extensions   myArray    1 2 3   eval set  a b c   myArray       Helper function   function set code        var vars code split      0  trim   split           var array code split      1  trim        return  var   vars map function x i  return x     array     i       join           Proof that it works in arbitrary scope    function        myArray    4 5 6       eval set  x y z   myArray         console log y       prints 5        eval is not supported in Safari

User · Answer

Here is a simple Javascript Tuple implementation   var Tuple    function         function Tuple Item1  Item2          var item1   Item1        var item2   Item2        Object defineProperty this   Item1               get  function     return item1                    Object defineProperty this   Item2               get  function     return item2                      return Tuple         var tuple   new Tuple  Bob   25      Instantiation of a new Tuple var name   tuple Item1     Assignment  name will be  Bob  tuple Item1    Kirk      Will not set it  It s immutable    This is a 2-tuple  however  you could modify my example to support 3 4 5 6 etc  tuples

User · Answer

I made a tuple implementation that works quite well  This solution allows for array destructuring  as well as basic type-cheking  const Tuple    function         function Tuple                Tuple needs at least one element         if  arguments length  lt  1                throw new Error  Tuple needs at least one element                       const args        arguments                Define a length property  equal to the number of arguments provided          Object defineProperty this   length                 value  arguments length              writable  false                         Assign values to enumerable properties         for  let i in args                Object defineProperty this  i                    enumerable  true                  get                         return args  i                                         Checking if the type of the provided value matches that of the existing value                 set value                        if  typeof value     typeof args  i                             throw new Error  Cannot assign     typeof value     on     typeof args  i                                               args  i    value                                                          Implementing iteration with Symbol iterator  allows for array destructuring as well for   of loops          this Symbol iterator    function                 const tuple   this               return                   current  0                  last  tuple length - 1                  next                         if  this current  lt   this last                            let val     done  false  value  tuple this current                             this current                            return val                        else                           return   done  true                                                                                  Sealing the object to make sure no more values can be added to tuple         Object seal this                 check if provided object is a tuple     Tuple isTuple   function obj            return obj instanceof Tuple                 Misc  for making the tuple more readable when printing to the console     Tuple prototype toString   function             const copyThis        this            const values   Object values copyThis           return     values join                          conctat two instances of Tuple     Tuple concat   function obj1  obj2            if   Tuple isTuple obj1      Tuple isTuple obj2                 throw new Error  Cannot concat Tuple with     typeof  obj1    obj2                       const obj1Copy        obj1            const obj2Copy        obj2             const obj1Items   Object values obj1Copy           const obj2Items   Object values obj2Copy            return new Tuple    obj1Items     obj2Items               return Tuple         const SNAKE COLOR   new Tuple 0  220  10    const  red  green  blue    SNAKE COLOR  console log green        gt  220

User · Answer

As an update to The Minister s answer  you can now do this with es2015   function Tuple    args      args forEach  val  idx    gt       Object defineProperty this   item  idx    get       gt  val            var t   new Tuple  a   123  console log t item0      a  t item0    b  console log t item0      a    https   jsbin com fubaluwimo edit js console

User · Answer

Tuples aren t supported in JavaScript  If you re looking for an immutable list  Object freeze   can be used to make an array immutable      The Object freeze   method freezes an object  that is  prevents new properties from being added to it  prevents existing properties from being removed  and prevents existing properties  or their enumerability  configurability  or writability  from being changed  In essence the object is made effectively immutable  The method returns the object being frozen    Source  Mozilla Developer Network - Object freeze    Assign an array as usual but lock it using  Object freeze     gt  tuple   Object freeze   Bob   24       Bob   24     Use the values as you would a regular array  python multi-assignment is not supported    gt  name   tuple 0   Bob   gt  age   tuple 1  24   Attempt to assign a new value   gt  tuple 0     Steve   Steve    But the value is not changed   gt  console log tuple     Bob   24

User · Answer

Javascript 1 7 added destructured assignment which allows you to do essentially what you are after   function getTuple       return   Bob   24     var  a  b    getTuple       a      bob    b     24 are both true

User · Answer

You have to do it the ugly way  If you really want something like this  you can check out CoffeeScript  which has that and a whole lot of other features that make it look more like python  sorry for making it sound like an advertisement  but I really like it

User · Answer

You can do something similar    var tuple   Object freeze   name  Bob   age 14      and then refer to name and age as attributes   tuple name  tuple age

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