javascript - Check if method exists in a class in TypeScript - Stack Overflow

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I am having a class in which a method currently doesn't exists but will be added later. I want to use that method if it exists otherwise not. For eg.

Class A {
    public func(string s) {
        // Currently this method is not present in Class A
    }
}

I want to check that if this method exists, then call it, otherwise do something else. I found a solution which works for JavaScript but doesn't work for TypeScript:

let objectOfA = new A();
if ( objectOfA.func === function) {
    objectOfA.func();
}

But this somehow doesn't works in TypeScript and throws pilation error saying Operator '===' cannot be applied to types 'string' and '() => any'.

Is there a way I can check for method existence in TypeScript?

I am having a class in which a method currently doesn't exists but will be added later. I want to use that method if it exists otherwise not. For eg.

Class A {
    public func(string s) {
        // Currently this method is not present in Class A
    }
}

I want to check that if this method exists, then call it, otherwise do something else. I found a solution which works for JavaScript but doesn't work for TypeScript:

let objectOfA = new A();
if ( objectOfA.func === function) {
    objectOfA.func();
}

But this somehow doesn't works in TypeScript and throws pilation error saying Operator '===' cannot be applied to types 'string' and '() => any'.

Is there a way I can check for method existence in TypeScript?

• javascript
• node.js
• typescript

Share Improve this question Follow edited Dec 13, 2020 at 18:15 PG1 asked Nov 15, 2017 at 7:42 PG1PG1 1,24222 gold badges1212 silver badges2828 bronze badges 3

• 1 Just ment out the function call as long as the function isn't implemented yet. – Cerbrus Commented Nov 15, 2017 at 7:45
• @Cerbrus Actually I want to check-in my code which should work before/ after implementation of the non-existent function. – PG1 Commented Nov 15, 2017 at 7:50
• 3 @Cerbrus Why is this closed as dublicate? This question is about TypeScrtipt and that one is about JS. TypeScript does add quite a bit of difficulty. If you try to check if non-existant function exists then the tools will yell that "such field doens't exist, don't do it" which exactly what you check. However when you override a method that may or may not be implemented it makes sence to check if its ancesor exists. – Gherman Commented Mar 4, 2020 at 12:26

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1 Answer 1

Sorted by: Reset to default 17

You can use typeof operator to get the type the of the operand. Although use bracket notation to get unknown properties(to avoid throwing exception).

let objectOfA = new A();
if ( typeof objectOfA['func'] === 'function') {
    objectOfA.func();
}

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