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170

How to create Javascript constants as properties of objects using const keyword?

rated 0 times [ 174] [ 4] / answers: 1 / hits: 67688 / 12 Years ago, fri, june 1, 2012, 12:00:00

How come constants cannot be set as properties of objects which are variables themselves?

const a  = 'constant' // all is well

// set constant property of variable object

const window.b = 'constant' // throws Exception

// OR

var App = {};  // want to be able to extend

const App.goldenRatio= 1.6180339887  // throws Exception

And how come constants passed by reference suddenly become variable?
EDIT: I know App won't (or rather... SHOULDN'T) be mutable; this is just an observation...

(function() {

    const App;

    // bunch of code

    window.com_namespace = App;

}());

window.com_namespace; // App

window.com_namespace = 'something else';

window.com_namespace; // 'something else'

How can a nicely organized, extensible, object-oriented, singly namespaced library containing constants be made with these limitations?

EDIT: I believe zi42, but I just have to ask why

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coby

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3 Years ago

answered 12 Years ago janettejordynm · Accepted Answer

You cannot do it with constants. The only possible way to do something that behaves like you want, but is not using constants, is to define a non-writable property:

var obj = {};

Object.defineProperty( obj, MY_FAKE_CONSTANT, {

  value: MY_FAKE_CONSTANT_VALUE,

  writable: false,

  enumerable: true,

  configurable: true

});

Regarding your question as to why a const passed to a function becomes variable, the answer is because it's passed by value and not by reference. The function is getting a new variable that has the same value as your constant.

edit: thanks to @pst for noting that objects literals in javascript are not actually passed by reference, but using call-by-sharing:

Although this term has widespread usage in the Python community, identical semantics in other languages such as Java and Visual Basic are often described as call-by-value, where the value is implied to be a reference to the object.