You are unable to call upon an object that may be of type 'undefined' in typescript

Among all the other inquiries on this topic, my issue lies with the typescript compiler seeming perplexed due to the following code snippet:

if(typeof this[method] === "function"){
    await this[method](req,res,next)
}

The error message I am encountering is:

Cannot invoke an object which is possibly 'undefined'
. I had assumed that typescript would be intelligent enough to recognize when typeguards are used to avoid such errors. I've attempted using ternary operators, if(this[method]), if(this[method]!==undefined), and they all resulted in the same error.

Providing some context:

for (const type of this.types){
    const method = type.toLowerCase() as Lowercase<Method>
    this.router[method](this.renderLocation, async (req, res, next) => {
        if(typeof this[method] === "function"){
            await this[method](req,res,next)
        }
    })
}

This block represents the entirety of the issue at hand, where Method is a type defined as follows:

export type Method =
    "POST" |
    "GET" |
    "PUT" |
    "DELETE"

Answer №1

When working with Typescript, it struggles to retain the context of this[method] in your type validation as you might expect. In fact, it treats each instance of this[method] as a completely new entity every time. It's akin to applying the .map method on an array without storing the outcome - technically valid but not very usable.

But fear not! Once you save the referenced method into a variable, Typescript will accurately remember the type check:

const m = this[method];

if (typeof m === "function") {
  m(req, res, next);
}

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