asynsPipe produces null as a first value

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When using async pipe in Angular that don't fire event immediately(http request or any observable with a delay), got first value as null Why is it happening? How to avoid that?

<hello [data]="delayedData$|async"> </hello>

First change:

SimpleChange {
   currentValue: null
   firstChange: true
   previousValue: undefined
}

Second change:

SimpleChange {
   currentValue: 'some real data'
   firstChange: false
   previousValue: null
}

Example on stackblitz: https://stackblitz.com/edit/http-async-pipe-crxm32

This is the expected behaviour. To avoid it you can use the *ngIf directive:

<hello *ngIf="delayedData$|async as delayedData" [data]="delayedData"> </hello>

This will only render the hello component only when the delayedData$ has emitted a value.

Angular docs on ngIf

JAVA - Performance with string concatenation in , asynsPipe produces null as a first value Key value in AWSIoTDataManager JavaScript: How can I get and use a value multiple times in an expression. But this example only works since a possible value for this.approved_by is the same as one of the potential values that you wish to set it to. For all other cases you will need to use the conditional operator as I showed in my first example.

Try like this:

<ng-container *ngIf="delayedData$ | async; let item">
  <hello name="Observable emitting with a delay" [data]="item" > </hello>
</ng-container>

Demo

What is a Database NULL Value?, A null value is used in databases to signify a missing or unknown value. A NULL can be I think doing that causes more confusion and would make it very easy to skew results. The first line evaluates to a number the others to NULL: 20 + (5� Using a not in clause when you compare a null value with a null value produces different results, depending on whether the variable or column contains the null values. This table shows the results compares between not null -value columns and null -value expressions using different comparison operators

you can simply use 'startWith' operator, as you are using in the one of your observable, the outcome is actually logical, since observable does not hold any initial value, it prints null, try operator startWith, or it's variation @Muhammed Albarmavi wrote down here (I would suggest to use startwith), I know you might have much complex data structure in your real life code but, you can pass empty array empty object etc. as well

B.4.4.3 Problems with NULL Values, Both statements insert a value into the phone column, but the first inserts a NULL An expression that contains NULL always produces a NULL value unless� asynsPipe produces null as a first value. Hot Network Questions Is it safe for plumbing to use rainwater to flush toilets? MySQL / MariaDB client slow to start/load

This is a known bug of an Angular, explained in this issue(marked as proposal, but it is a bug) https://github.com/angular/angular/issues/16982

Possible solutions are listed in this discussion. In general the solution is to use startWith rxjs operator, use ?. in templates.

Also there is a tslint rule for preventing situations like that http://codelyzer.com/rules/template-no-negated-async/

FIRST_VALUE, It returns the first value in an ordered set of values. If the first value in the set is null, then the function returns NULL unless you specify IGNORE NULLS . 1.A NULL value represents the absence of a value for a record in a field (others softwares call it also a missing value). 2.An empty value is a "field-formatted" value with no significant data in it. 3.NULL isn't allocated any memory, the string with NUll value is just a pointer which is pointing to nowhere in memory. however, Empty IS

a simply whay to provide a default value until the request is done

<hello 
name="Observable emitting with a delay" 
[data]="((delayedData$|async) || 'hello from earth 🌍')"> 
</hello>

demo ⚡

More Tips and Tricks - Guide, For example, often you want to replace null values with zero: data = FOREACH Or, sometimes you want to return the first non-null value among several fields: It relies on the fact that flattening an empty bag produces no output. Therefore� The IS NULL condition is satisfied if the column contains a null value or if the expression cannot be evaluated because it contains one or more null values. If you use the IS NOT NULL operator, the condition is satisfied when the operand is column value that is not null, or an expression that does not evaluate to null.

If a NULL value is written to a character type column, both the SAS/SHARE driver and the IOM driver interpret the value as a SAS missing value and return FALSE for wasNull. Note: If a numeric type is set to NULL, wasNull returns TRUE as expected.

Null values in arithmetic operations yield different results depending on the type of data source. In Microsoft SQL Server Analysis Services (SSAS) and Cubing Services data sources, a null value in arithmetic operations is treated as follows: In operations such as addition (+), a null value yields the same result as zero (9 + NULL = 9)

Comments
  • Isn't that the expected behavior judging from the implementation of the async pipe?
  • you can simply use 'startWith' operator, as you are using in the one of your observable, the outcome is actually logical, since observable does not hold any initial value, it prints null, try operator startWith, or it's variation @Muhammed Albarmavi wrote down here (I would suggest to use startwith), I know you might have much complex data structure in your real life code but, you can pass empty array empty object etc. as well
  • If you dive into the implementation – you can expect it, but there is nothing in official dos, and IMHO it's very strange behaviour – I'm expecting only the values from my observable, without sideeffects
  • this is a nice solution, but the problem is not to render element only when data comes, but be sure that what am I getting in an input is a data that i got from backend.
  • @KaterinaPavlenko but the problem is not to render element only when data comes? I do not understand that statement. Can you clarify what you mean.
  • your solution let me render the component only when delayedData$ produces not-null value. It is good, but what if delayedData$ can produce null values and it is the normal behaviour of it? And I want to render component not depending on value produced by delayedData$?
  • long story short – I want to work with @Input data in ngOnChanges() and divide default null from real null
  • this is nice idea, but the solution from this answer is shorter, doing the same stackoverflow.com/a/59249460/4166537
  • Yep, I was going to add this as another option, but since I saw someone already added it, I didn't mention :)
  • aggred the startWith is the correct one and other answers is just a work arround 👍
  • this might be solution as well, but it behaves absolutely same as startwith operator, but start with opereator is much cleaner way to achieve same result
  • @LashaSumbadze yes it just an alternative way but I think all answer can work very well 👍