What is the differnce between 'do' and 'finally' when using RxJS observables to take action after the observable returns?

Question

I don't see very good documentation on what the difference is between do and finally in RxJS. My goal is to take action only when an Observable returns data but it looks like they will both take action on failure as well.

• do says "Invokes an action for each element in the observable sequence and invokes an action upon graceful or exceptional termination of the observable sequence."

The observable might return more than one element?

• finally says "Invoke a specified action after the source observable sequence terminates gracefully or exceptionally".

My hope is that someone will explain if it matters which is used or if there is a better alternate method.

    getData(choice): void {
        this.dataService.getTableData(choice, 'mainCalls.php')
            .do( () => this.defineWidth() )
            .subscribe(tableData => this.tableData = tableData,
                err => {
                    console.log(err);
                }
            );
    }

    ngOnInit() {
        this.getData('getTableData');
    }

defineWidth is a function that is dependent upon the data being returned by the Observable. I'm open to suggestion and reading material on alternate methods to accomplish what I want.

Answer 1

do() is called for each normal event and does not modify the data stream. It is used only for side effects.

finally() is called once after the last event or after an error if any. It is called once in either case success or failure.

If this.defineWidth() depends on this.tableData than you don't need do or finally. Just add the call after the line where you assign the response to this.tableData:

getData(choice): void {
    this.dataService.getTableData(choice, 'mainCalls.php')
        .subscribe(tableData => {
           this.tableData = tableData;
           this.defineWidth();
         }),
         err => {
           console.log(err);
         }
    );
}