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How To Access Private Class Members in TypeScript

I know we shouldn't access private class members. But sometimes, there is simply no way around. For instance, during unit tests we can access private members to assert the correct values. Or while debugging we want to log a private property to the console.

In TypeScript there are two ways to do this. The first option is to cast the object to any. The problem with this option is that you loose type safety and intellisense autocompletion. The second option is the intentional escape hatch. This is a bug (or feature?) in Typescript which was left open for this very purpose. It allows access to private members via bracket notation while maintaining type safety and autocompletion.

class Test {
    public foo: string = "Foo";
    private bar: string = "Bar";
    public constructor() { }

const test = new Test();

// cast as any
console.log((test as any).bar)

// intentional escape hatch
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Check it yourself at TypeScript Playground.

Top comments (1)

benhultin profile image
ben hultin • Edited

Accesing private properties even for testing purposes I would not recommend.

Testing private properties should be through the public methods / functions which those interact with the private property to achieve the final outcome.

This is the point of integration testing which verifies the flow from start to finish involving multiple methods working together.

Note: as you mentioned about refraining from using any, the second method still makes use of any. As the phrase goes, "anything is better than any"