javascript : dictionary of functions with "constant" integer as keys

Question

The following code is working fine:

function a() { console.log("a"); };
function b() { console.log("b"); };

var F = {1:a,2:b};

F[1](); // log "a"
F[2](); // log "b"

but this one does not:

function a() { console.log("a"); };
function b() { console.log("b"); };

var A = 1;
var B = 2;
var F = {A:a,B:b};

F[A](); // log "TypeError : F[A] is not a function"

The idea is that A and B would be constant integer, used just for the sake of code readability. I know there are no "const" in javascript, but you get the idea.

What would be the clean way to get something like this work ? (I do not care that much if A and B are effectively constant or not at the end, just having them upper case indicates my future me not to change their value).

possible duplicate of [Using a variable for a Javascript object key](http://stackoverflow.com/q/2274242/1048572)? — Bergi, Nov 18 '14 at 05:58

score 15 · Answer 1 · 2015-07-02T11:15:15.313

15

In ES6, use computed property names:

{ [A]: a }

edited Jul 02 '15 at 11:15

answered Nov 18 '14 at 05:09

That has to be the shortest usable answer I ever read here :-) – Pascal May 10 '21 at 19:58
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#computed_property_names – Jacktose Feb 17 '22 at 22:53

score 3 · Accepted Answer · answered Nov 18 '14 at 05:04

3

You're using A and B as identifiers in the JavaScript object you're creating, they aren't referencing the variables you've declared.

Instead you'll have to add them to the JavaScript object like so:

function a() { console.log("a"); };
function b() { console.log("b"); };

var A = 1;
var B = 2;
var F = {};
F[A] = a;
F[B] = b;

F[A]();

answered Nov 18 '14 at 05:04

Aaron Powell

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I am quite confused, thought my code and your code was totally equivalent ... My first batch of code in JavaScript comes with a lots of surprises – Vince Nov 18 '14 at 05:07

score 2 · Answer 3 · answered Nov 18 '14 at 05:36

Within an object literal, all the "keys" are interpreted as strings, with or without quotes. So your key is "A", not 1. Your example is equivalent to var F = { "A": a, "B": b }.

Unfortunately, you simply cannot use a variable for a key within an object literal.

The only way to set a variable key is to use bracket notation:

F[A] = a;

javascript : dictionary of functions with "constant" integer as keys

3 Answers3