Error occurs when attempting to test both boolean and number data within an ngIf statement

In the scenario where I am working with a template that includes a boolean called readOnly and an array known as arrayOfStuff:

<span *ngIf="!readOnly && arrayOfStuff && arrayOfStuff.length">Hey</span>

When running eitherng build --prod orng serve --prod, an error is thrown:

ERROR in /Development/project/src/$$_gendir/app/components/thing/thing.component.ngfactory.ts (767,11): Type 'number' is not assignable to type 'boolean'.

However, if I remove the boolean check, the code works perfectly fine:

<span *ngIf="arrayOfStuff && arrayOfStuff.length">Hey</span>

Alternatively, if I specifically compare the length of arrayOfStuff as a number, it also resolves the issue:

<span *ngIf="!readOnly && arrayOfStuff && (arrayOfStuff.length >0)">Hey</span>

Why does checking against the falsy value of arrayOfStuff.length work when determining the existence of an object, but not when evaluating a boolean value together with it?

It is important to note that this issue only occurs during production builds using ng build --prod or ng serve --prod. It does not show up in the development environment.

To clarify - although the problem can be worked around, understanding why direct checks on arrayOfStuff.length behave differently in these instances is crucial.

Using Angular v4 and @angular/cli v1.0.1.

angulartypescriptangular-cliangular2-aot

Answer №1

When you choose not to use comparators in JavaScript, the language attempts to coerce types on variables. In this scenario, when it is unable to directly convert to a boolean, an error occurs. However, by utilizing

array.length > 0

this issue is avoided since all variables are already boolean. It seems the difficulty lies in changing primitive numbers to booleans. To achieve the desired outcome, you can employ

!!array.length

to successfully obtain your result

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