Define prototype function with typescript

Question

When I try to define a prototype function, I get:

error TS2339: Property 'applyParams' does not exist on type 'Function'.

Function.prototype.applyParams = (params: any) => {
     this.apply(this, params);
}

How to solve this error?

Try this: stackoverflow.com/a/28020863/1142380 I don't think you need the "prototype." part — ToastyMallows
– ToastyMallows, Commented Jan 20, 2017 at 22:13
@ToastyMallows but then I get error TS2339: Property 'applyParams' does not exist on type 'FunctionConstructor'. Even with interface FunctionConstructor { applyParams(params: any): any; } — Alexandre
– Alexandre, Commented Jan 20, 2017 at 22:19

David Sherret · Accepted Answer · 2019-01-08 14:45:42Z

48

Define the method on an interface named Function in a .d.ts file. This will cause it to declaration merge with the global Function type:

interface Function {
    applyParams(params: any): void;
}

And you don't want to use an arrow function so that this won't be bound to the outside context. Use a regular function expression:

Function.prototype.applyParams = function(params: any) {
    this.apply(this, params);
};

Now this will work:

const myFunction = function () { console.log(arguments); };
myFunction.applyParams([1, 2, 3]);

function myOtherFunction() {
    console.log(arguments);
}
myOtherFunction.applyParams([1, 2, 3]);

edited Jan 8, 2019 at 14:45

answered Jan 20, 2017 at 22:19

David Sherret

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5 Comments

Alexandre Over a year ago

I also tried to use an interface and it still get the error. I tried interface Function and interface FunctionConstructor

David Sherret Over a year ago

@Alexandre are you using external modules? Define the interface in a definition file (.d.ts file) and reference that in your application

David Sherret Over a year ago

@Alexandre you can see this working here

Alexandre Over a year ago

Thanks for the .d.ts advice

Sebastian Nielsen Over a year ago

@DavidSherret "Define the interface in a definition file (.d.ts file)", could you elaborate on how to do that?

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