iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
Angular NG8101 means a two-way binding uses the delimiters in the wrong order: ([user]) instead of [(user)]. Move the parentheses inside the square brackets. The reversed form is treated as an event binding named [user], so it does not create the intended two-way binding.
What NG8101 means
Angular’s “banana-in-a-box” syntax combines property binding ([]) with event binding (()) to pass a value to an element and let changes flow back. The correct combined delimiters are [()]: the parentheses go inside the square brackets. NG8101 flags the backwards form, ([name]).
As Angular explains, “As it stands, ([var]) is actually an event binding with the name [var].” Angular’s NG8101 reference describes the diagnostic.
Quick wins for a faster PC:
Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →How to fix NG8101
Change the delimiter order in the template. For example:
#1 Best Overall
<!-- Incorrect: backwards delimiters -->
<user-viewer ([user])="loggedInUser" />
<!-- Correct: parentheses inside the square brackets -->
<user-viewer [(user)]="loggedInUser" />Apply the same correction to the binding that triggered the diagnostic: replace ([name]) with [(name)]. The corrected syntax expresses the intended two-way binding, provided the element or component supports that binding.
What two-way binding does
With a two-way binding, a value is passed into an element and changes made by that element can update the bound component data. For example, Angular’s template-driven forms use [(ngModel)] on a form control when user input should update component data. See Angular’s guides to two-way binding and form input validation.
When Angular reports NG8101
NG8101 is an Angular template diagnostic for the reversed binding delimiters. It is not a missing-package error or a runtime form validator; fix the binding syntax in the template.
Extended diagnostics require strictTemplates to be enabled. The NG8101 check has no additional prerequisite stated in its diagnostic reference. If the diagnostic is not appearing, check the template type-checking setting in the project’s Angular compiler configuration.
Rank #3
Configure the diagnostic only if needed
Extended diagnostics are warnings by default. In angularCompilerOptions, an individual check can be configured as warning, error, or suppress. Angular documents this configuration for suppressing NG8101:
{
"angularCompilerOptions": {
"extendedDiagnostics": {
"checks": {
"invalidBananaInBox": "suppress"
}
}
}
}Use suppression only when there is a reason to silence the warning; correcting the delimiter order is the direct fix. The Extended Diagnostics overview explains the available categories and the defaultCategory setting, which applies to checks not individually configured. Setting a check to error makes it fail compilation; suppressing it disables that diagnostic. Be cautious about making all extended diagnostics errors by default: Angular notes that new diagnostics may be added or enabled in minor releases, potentially turning a newly surfaced check into a compilation failure.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
