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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 so the binding can pass a value to the element and receive updates from it.
What NG8101 means
Angular reports NG8101 when it encounters reversed “banana-in-a-box” syntax in a template. For example:
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<user-viewer ([user])="loggedInUser" />This does not create a two-way binding. Angular interprets ([user]) as an event binding whose event name is [user]. As Angular puts it, “As it stands, ([var]) is actually an event binding with the name [var].” Angular’s NG8101 diagnostic explains the error.
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Put the parentheses inside the square brackets. Change ([user]) to [(user)]:
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<!-- Incorrect: reversed delimiters -->
<user-viewer ([user])="loggedInUser" />
<!-- Correct: parentheses inside square brackets -->
<user-viewer [(user)]="loggedInUser" />The corrected form is Angular’s two-way-binding syntax: it passes the value to the element and allows the element to propagate changes back. The [()] delimiters combine property binding, [], with event binding, (). One familiar use is [(ngModel)] on a form control whose user-entered value should update component data; see Angular’s two-way binding guide and form input validation guide.
When Angular reports NG8101
NG8101 is an Angular template diagnostic for a binding typo, not a missing package or a runtime form-validation error. Extended diagnostics require strictTemplates to be enabled. The invalid banana-in-box check has no additional prerequisite listed in Angular’s diagnostic reference.
Configure the diagnostic only if needed
Extended diagnostics are warnings by default. In angularCompilerOptions, a project can set an individual check to warning, error, or suppress. For this check, the configuration key is invalidBananaInBox. Angular documents suppressing it as follows:
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"angularCompilerOptions": {
"extendedDiagnostics": {
"checks": {
"invalidBananaInBox": "suppress"
}
}
}
}Suppression hides this diagnostic; it does not correct the reversed binding. Fixing the delimiter order is the direct solution. If you configure diagnostic severity, note that an error blocks compilation, while a warning does not. Angular also cautions that new diagnostics may be added or enabled in minor releases, so setting defaultCategory to error can make a newly surfaced diagnostic fail a build. See the Extended Diagnostics overview for configuration details.
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