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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Angular’s ExpressionChangedAfterItHasBeenCheckedError (NG0100) means a template-bound value changed after Angular had already checked it. Angular throws this error only in development mode, where an additional check helps reveal bindings that do not stay stable. Find the expression that changed, then fix when or where its value is updated.
What NG0100 means
During a change-detection traversal, Angular checks each component once as it moves through the component tree. A binding that changes after its check leaves the view inconsistent for that pass; the development-mode verification check reports the change as NG0100. The error is a signal to inspect the state flow, not a diagnosis of one particular bug. Angular’s NG0100 guide explains the error and its development-mode scope.
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How to find the expression that changed
- Read the complete error. Note any expression, previous and current values, component, or template location included in the message.
- Follow the source-mapped stack trace. With Angular CLI source maps enabled, use the call stack to trace the reported change back to the component and template.
- Inspect the binding and every writer. Search for assignments to the property and for methods, getters, subscriptions, callbacks, or child components that can change it. Check when each runs relative to the component’s change-detection pass.
- Make template reads stable. A method or getter used by a template should return a stable value for the same state and should not mutate state that other bindings depend on.
Fix the update at its source
Initialize state before the template check
For initial component state, use the constructor or ngOnInit where appropriate. Angular calls ngOnInit after the initial inputs are initialized and before the component’s template is initialized. Use ngOnChanges when deriving state from input changes; it runs before the component’s own template is checked. See the Angular lifecycle guide for the current hook ordering.
Avoid changing checked state in after-content and after-view hooks
ngAfterContentInit, ngAfterContentChecked, ngAfterViewInit, and ngAfterViewChecked are intended for work involving initialized content or view queries. Mutating template-bound state in these hooks can trigger NG0100 because the relevant view has already been checked. The checked hooks also run repeatedly, so using them to force another update can add unnecessary work. Prefer moving initialization earlier or redesigning the state flow.
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Correct child-to-parent updates
If a child changes a value that its parent has already used in the current traversal, the parent’s binding may no longer match what Angular checked. Trace the update to the component that owns the state and arrange for the value to be supplied at an appropriate point in the flow, rather than mutating an already-checked parent from a child hook.
Use computed state for signal-derived values
When one signal-derived value depends on other state, use computed to express that relationship. Angular cautions against using effect to propagate state: effects can create NG0100, circular updates, or unnecessary change-detection cycles. See Angular’s effects guide.
In zoneless apps, ensure updates notify Angular
In a zoneless application, changing a value is not by itself a guarantee that Angular will schedule the needed view update. Use an update mechanism that notifies Angular, such as updating a signal read by the template; where appropriate, ChangeDetectorRef.markForCheck() can mark a view for checking. The zoneless guide describes notification options. A missing notification can cause an update to go undetected in production; it is a distinct concern from an NG0100 error, which Angular documents as development-mode-only.
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Angular documents a forms-specific behavior: when a value change originates in an input binding, NgModel triggers a second change detection to avoid this error. That behavior belongs to NgModel; it is not a general-purpose pattern to copy into unrelated components. Likewise, delaying a mutation or triggering extra checks without correcting the underlying timing can hide the symptom while leaving state flow fragile.
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Version scope
The current Angular documentation describes NG0100 as a development-mode error. Error details and behavior can vary across Angular releases and configurations, so use documentation for the version your application runs rather than assuming all historical releases behave identically.
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