How to get data type from string javascript

Question

I have a few strings

strings = [
    "String",
    "Object",
    "Boolean"
]

and I would like to use them to declare new objects

for(var i = 0; i < strings.length; i++){
    var x = new strings[i];
}

but instead I get this error:

TypeError: string is not a function
  at Object.<anonymous> (/Users/path/code.js)
  at Object.<anonymous> (/Users/path/code.js)
  at Module._compile (module.js:456:26)

How can I treat strings as their underlying types?

Aside:

I know that I could do something like

type_map = {
    String: String,
    Object: Object,
    Boolean: Boolean
}

and then go

for(var i = 0; i < strings.length; i++){
    var x = new type_map[strings[i]];
}

which works, but I'm looking for something slightly more elegant if it exists.

Right, Perhaps I'm not phrasing this correctly, but what I'm trying to find out if there is some way to get the literal from the String string — Loourr
– Loourr, Commented Nov 25, 2014 at 23:25
"new" can be used only on function, not on string. All global variables are elements of "window", so window["String"] corresponds to global function String. — Ivan Kuckir
– Ivan Kuckir, Commented Nov 25, 2014 at 23:48

Alin P. · Accepted Answer · 2014-11-25 23:27:38Z

3

What's wrong with the obvious?

var i, x, constructors = [
    String,
    Object,
    Boolean
];

for(i = 0; i < constructors.length; i++){
    x = new constructors[i];
}

Bear in mind that anything in JS is an object (including "class names") and may be used, pretty much, however you want.

answered Nov 25, 2014 at 23:27

Alin P.

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Comments

guest · Accepted Answer · 2014-11-25 23:26:46Z

1

The constructors for these classes are properties of the global object.

For example, in a browser, you could do

console.log(window['String'] === String); // true
var str = new window[strings[0]]();
console.log(str); // ""

answered Nov 25, 2014 at 23:26

guest

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Comments

Loourr · Accepted Answer · 2014-11-26 16:49:51Z

0

Found a great way to do it with the eval function

strings = [
    "String",
    "Object",
    "Boolean"
]

for(var i = 0; i < strings.length; i++){
    var x = new (eval(strings[i]));
}

so to summarize eval("String") === [Function: String]

edited Nov 26, 2014 at 16:49

answered Nov 25, 2014 at 23:31

Loourr

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5 Comments

Alin P. Over a year ago

Actually I think you found the one wrong way to do it. Please consider the other options before going to the eval, as you will spoil yourself with bad pratices.

Alin P. Over a year ago

@BatScream Yeah, there's technically no problem. The eval will evaluate just the string and return the function contained in the object with the name specified by the string. Then will apply the function as a constructor on the context created by new.

BatScream Over a year ago

@AlinPurcaru -Thanks for the explanation. Got a clear picture. There is a syntax error there though i think. new (eval(strings[i]))

jfriend00 Over a year ago

Much better to use Alin's solution than using eval() as eval() is slow and generally something to shy away from unless there is no other solution, but usually, there is another solution.

Alin P. Over a year ago

@BatScream It seems so. The new operator is implemented a bit differently than I imagined, but the error can be circumvented with some custom operator priorities: new (eval(strings[i])). Disclaimer: This comment is just for sport, please don't use this code in software development.

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