Convert string name of JS Object to instance of JS Object

Question

I have a function, which receives 2 params: a variable, and a string representation of a data type ('String', 'Object' etc):

function typeChecker(variable, dataType) {
    // checks type, returns true or false
}

I want to convert the second parameter to a constructor, so that this expression does not throw an error:

variable instanceof 'Date'

Question: Is it possible to convert any of these:

'String'
'Date'
'Object'

To these:

String
Date
Object

Does the dataType have to be a string, because parameters can be constructors, so typeChecker({}, Object) is possible — Sebastian Speitel
– Sebastian Speitel, Commented Aug 16, 2018 at 18:33
@SampsonCrowley If it is, how to figure it out in order to throw an error? — Eduard
– Eduard, Commented Aug 16, 2018 at 18:34
@Eduard you'd have to only accept params from a whitelist. Using bracket notation on window or global is a better option — Sampson Crowley
– Sampson Crowley, Commented Aug 16, 2018 at 18:36
@SebastianSpeitel I was thinking about that, but have not dared to implement this way, trying to be cautious in order not to end up in some unexpected behavior. Besides, for primitive data types I use typeofsince was not sure about performance implications of using instanceof when not necessary. — Eduard
– Eduard, Commented Aug 16, 2018 at 18:37

apsillers · Accepted Answer · 2018-08-16 19:33:15Z

3

Those constructors all happen to be members of the global object (either window in the browser or global in Node.js), so you could do one of

variable instanceof window['Date']
variable instanceof global['Date']

If your constructor does not exist as a member o the global object, you can check if any prototype in the value's prototype chain is associated with a constructor whose name matches the desired string:

function checkIfValueIsOfTypeName(value, typeName) {
    while(value = Object.getPrototypeOf(value)) {
        if(value.constructor && value.constructor.name === typeName) {
            return true;
        }
    }
    return false;
}

This is more or less how instanceOf operates internally, except instanceOf directly compares constructor to the right-hand value, instead of comparing its name to a string, which is what you want to do.

edited Aug 16, 2018 at 19:33

answered Aug 16, 2018 at 18:34

apsillers

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2 Comments

Eduard Over a year ago

I assume it is safe to use window object in this case?

apsillers Over a year ago

@Eduard Yes, the window object is safe to use both in terms of availability (it is globally available) and security (unlike eval which could cause arbitrary code execution that you didn't intend).

regex · Accepted Answer · 2018-08-16 18:39:38Z

0

You can use typeof

console.log(typeof 10);
// output: "number"

console.log(typeof 'name');
// output: "string"

console.log(typeof false);
// output: "boolean"

answered Aug 16, 2018 at 18:39

regex

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1 Comment

Eduard Over a year ago

Not for composite data types. This is why ìnstanceof is used in the question.

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