I'm designing some classes and for some data I want to use fields that are constant.
Now, AS USUAL, IE does not support the key const so
const T = 10;
not work.
I could create a property via __defineGetter__ and __defineSetter__ to mime that but
, AS USUAL, IE does not support that syntax (IE 8 has Object.defineProperty but not work on custom object).
Any idea?
"No solution" is really the safest solution. Right now there is not a good mechanism for creating constants in Javascript, but if you establish a convention (ALL CAPS FOR CONSTS) to help identify them and do not overwrite them yourself then your own constants will be safe.
If you are writing a public library and you are really worried about someone modifying a value then you can use a private variable only accessible to your own code by controlling scope with a function:
(function() {
var MY_CONSTANT = 42;
alert(MY_CONSTANT); // alerts "42"
...do other stuff in here...
})();
alert(MY_CONSTANT); // alerts "undefined" because MY_CONSTANT is out of scope here
If a developer can read a value in javascript then that developer can modify that value.
There is a freeze() method in Javascript which can be used to create actual constants.
Usage:
var consts = {}; // Object to store all constants
consts.MY_CONSTANT= 6;
Object.freeze(consts); // Make the consts truly read only
consts.MY_CONSTANT= 7; // Trying to do this will give error
Object.defineProperty is what you want.
For Example:
var obj = {};
Object.defineProperty(obj, 'key', {
enumerable: false,
configurable: false,
writable: false,
value: 'static'
});
Wil define "obj.key" with value : "static" and it will be readonly.
In essence, by putting them all to false, you are creating constants on the object. Btw, their defaults is to be at false.
So effectively doing this:
Object.defineProperty(obj, 'property', { value: 'value'});
Will create a constant on the 'obj' called 'property' with value 'value'. Or you could do this
function setConstant(obj, key, value)
{
Object.defineProperty(obj, key, {value: value });
}
var obj = {};
setConstant(obj, "constantName", "constantValue");
Would make it very readable.
Old question but nowadays you can simply use:
const MY_CONST = 10;
From MDN:
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.
Supported by all the main browsers already.
Note that defining something as a constant really just ensures that it can't be overwritten.
If you really can't define something as a constant, what about just defining it as a variable and setting its value once. I assume you're writing the code, so you can make sure you don't overwrite it...
Not ideal obviously, but it should work.
