How to access class variable inside Promise then() function? [duplicate]

Question

I'm working on a Angular4 + PHP website, where i send HTTP requests to the server using Promise, since i want my application to perform routing according to the response of the server. I want to access a class variable inside the then() but it throws undefined() error.

Here is my code:

status = false;

checkUser(): Promise<any>{
// .....
    return this.http
        .get('http://localhost:8000/api/user', this.options)
        .toPromise()
        .then(this.extractData)
        .catch(this.handleError);
}

private extractData(res: any) {
    data = res.json();
    this.status = data.status; // here it throws error undefined 
    return this.status; 
}

Is there any other way to implement this?

End

Again. Change this.extractData to res => this.extractData(res). This question is being asked again and again. – Concenter

@JB Nizet Yes, I do agree, its been asked quite many times, but as a beginner to angular,typescript its very hard to follow up others code, hope you can understand :) – End

Sure. I've done this mistake myself. But google is your friend. Googling for the title of your question and clicking on the first few results would have given you the answer. – Concenter

@JB Nizet Thank you! Yeah I'll follow it :) – End

Answer 1

If you pass a function reference, this won't point to the local class instance anymore.

You can use bind

.then(this.extractData.bind(this))

or arrow functions

.then((res) => this.extractData(res))

to get the desired behavior.

Cesura

Thanks for the answer :). It works – End