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Compiling TypeScript with filter () thisArg

I am working on an application in Node written in TypeScript and I would like to use filter()

to filter an array of objects based on an attribute. I have a public method ( getValidObjects()

) that takes an object ( preferred

) that has properties that I would like to map.

In my current setup I am using thisArg

to set the context in my callback to the preferred object.

class Sample {

    _objects:Object[];
    _preferred:Object;

    constructor() {
        this._objects = [
            {
                valid: true,
                label: 'This is valid'
            },
            {
                valid: false,
                label: 'This is invalid'
            }
        ];
    }

    public getValidObjects(preferred:Object) {
        return this._objects.filter(this.filterObjects, preferred);
    }

    private filterObjects(sample:Object, index:number, array:Object[]) {
        // "this" should be the preferred object

        return this.valid == sample.valid;
    }

}

export = Sample;

Ultimately the class compiles, but at the end it throws an error:

error TS2339: Property 'valid' does not exist on type 'Sample'.

It looks like the compiler is choking because it is trying to do type checking on the class with this

. I'm not sure if this is a bug tsc

, and she just doesn't know how to handle it thisArg

, or if there is another way to accomplish this.

Additional Information

  • I am using typescript-require for require

    my modules;
  • This is not my exact implementation, but it is close enough to illustrate the problem.

TL; DR

How can I get tsc

to compile my class without throwing an error when using the parameter filter()

thisArg

?

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1 answer


This error occurs because you have created a class level method filterObjects()

and are using it in a different context.

TypeScript compiler does not know the intended use of this method, he decides that you are going to use it in the context of Sample

: sample.filterObjects()

.

You have at least 2 different options for fixing this error:



  • Insert this

    into the type you have in your array:

    private filterObjects(sample:Object, index:number, array:Object[]) {
    // here, Object is your type, but it can be anything else
    // you have to cast to <any> first, because compiler may warn you
    // that Sample can not be cast directly to your type

    return (<Object><any>this).valid == sample.valid;
}

  • Move the filterObjects()

    declaration inside getValidObjects()

    :

    public getValidObjects(preferred:Object) {
    function filterObjects(sample:Object, index:number, array:Object[]) {
        return this.valid == sample.valid;
    }
    return this._objects.filter(filterObjects, preferred);
}

I suggest the second one, it is semantically better (if filterObjects

not a Sample

method, it shouldn't be on it).

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