Checking boolean truth value in Angular HTML

Question

I am trying to check a Boolean values truthiness in Angular HTML, and getting an odd result. After reading this SO post I thought I understood what was happening. I expected to be able to use the === operator, but as my last example below shows, that does not return the correct result.

StackBlitz example of my code

The output states of my code above:

nothing selected, but default val is false

create new: false

double eq isCreateNew==false: true

string isCreateNew=='false': false <----- unexpected

obj isCreateNew===false: true

set to true create new: true

double eq isCreateNew==false: false

string isCreateNew=='false': false

obj isCreateNew===false: false

set to false

create new: false

double eq isCreateNew==false: false <----- unexpected

string isCreateNew=='false': true

obj isCreateNew===false: false <----- unexpected

score 2 · Answer 1 · edited Jun 20 '20 at 09:12

As per the type conversion rules, a string 'false' does not get converted to the boolean false. This means that

false == 'false' // It is indeed false

So this should not be unexpected, it is the expected result.

Now you might ask why it returns true when you set isCreateNew to false. The problem is actually in your code. Your radio buttons are not setting the value to boolean false or true rather it is setting it to a string. To set it to a boolean, you need to use square brackets.

<mat-radio-button class="example-radio-button" [value]="true">set true</mat-radio-button>
<mat-radio-button class="example-radio-button" [value]="false">set false</mat-radio-button>

Another way to tell your value is not being set properly is by your ngModel. If you set isCreateNew to false in your component, it should set check the false radio button. Since your values are strings, this will not happen. Once you add the square brackets, you can see that the false radio button gets checked.

the link of **type conversion rules** is a grate hidden treasure of javascript — Muhammed Albarmavi, Sep 18 '19 at 16:04

score 1 · Accepted Answer · answered Sep 18 '19 at 15:50

Only an empty string equals `false' in JS. So, the expected result is that

string isCreateNew=='false': false

always. You can easily check this in the browser console.

Then, you don't use square brackets when binding the mat-radio-button value property. In this case, the passed value is not evaluated as JavaScript. As a result, the value of your mat-radio-button elements is a string. Not a boolean. Use square brackets to resolve other issues.

<mat-radio-group class="example-radio-group" [(ngModel)]="changeAE">
    <mat-radio-button class="example-radio-button" [value]="true">set true</mat-radio-button>
    <mat-radio-button class="example-radio-button" [value]="false">set false</mat-radio-button>
</mat-radio-group>

Muhammed Albarmavi · Answer 3 · 2019-09-18T16:06:27.357

like this value="true" will set changeAE to the value of 'true' as string but this [value]="true" changeAE to true boolean value

<mat-radio-group class="example-radio-group" [(ngModel)]="changeAE">
    <mat-radio-button class="example-radio-button" [value]="true">
      set true
    </mat-radio-button>
    <mat-radio-button class="example-radio-button" [value]="false">
      set false
    </mat-radio-button>
</mat-radio-group>

=== is compare and check the type of comparison sides typeof 'true' is string and typeof true is boolean if any of the type

Checking boolean truth value in Angular HTML

3 Answers3