Apart from eval(`o = new ${className}(${args.join(", ")})`), is there any other way to instantiate an object using variable argument list?
E.g.: var foo = instantiate(className, [arg1, arg2, ...])
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3 Answers
You can instantiate an object with a variable argument list like this:
function instantiate(className, args) {
var o, f, c;
c = window[className]; // get reference to class constructor function
f = function(){}; // dummy function
f.prototype = c.prototype; // reference same prototype
o = new f(); // instantiate dummy function to copy prototype properties
c.apply(o, args); // call class constructor, supplying new object as context
o.constructor = c; // assign correct constructor (not f)
return o;
}
Side note: you may wish to pass a direct reference to the class constructor function:
var foo = instantiate(Array, [arg1, arg2, ...]);
// Instead of:
var foo = instantiate("Array", [arg1, arg2, ...]);
... which makes this compatible with non-global functions.
5 Comments
Can Poyrazoğlu
window[className] is undefined for me (yeah, triple checked my class name).
JavaSheriff
TypeError: c is undefined because (window[className] is undefined )
Hiranmoy Chatterjee
Instead of c.apply(o, args), I think it should be c.constructor.apply(o, args). Else error occurs as "Class constructor must be invoked with 'new' operator.".
César Rodriguez
Probably if you are using node (not doing it inside of a browser) the window object will be undefined...
Pablo Pazos
window works in ES5 but not on ES6, the answer date is from 2011, don't expect that to work today...
Using Object.create() in ES5:
function instantiate(constructor, args) {
var instance = Object.create(constructor.prototype);
constructor.apply(instance, args);
return instance;
}
Using the spread operator in ES6:
var foo = new constructor(...args);
9 Comments
Ярослав Рахматуллин
Something tells me this will be four times faster than the accepted answer.
Alexey Lebedev
@inetphantom the first or the second one?
Hoshi
Downvoted because this answer requires using a type literal, which is antithetical to the question’s premise.
Alexey Lebedev
@user1944491 what even is a "type literal"?
Hoshi
@AlexeyLebedev In the statement
let m = new Map() the word Map is a type literal. You can clearly see in the OP’s question that he wants dynamic types, not static predetermined types. |
Well you can always do as follows. Anything added to Dino prototype can be shared among the instantiated objects The difference from normal constructor pattern is, the instantiated objects do not have to have the exact same private properties set. They can be set dynamically for each one of them.
function Dino(a,b){
for(i = 0; i< a.length; i++) this[a[i]] = b[i];
}
var props = ["foo", "bar"],
values = [42, 37],
obj = new Dino(props,values);
console.log(obj);