The error [$interpolate:interr] Error evaluating expression in AngularJS occurs when there is an issue with the interpolation of expressions within your HTML. This typically happens when AngularJS is unable to evaluate or parse an expression in the context of its interpolation system.
In AngularJS, interpolation is the process of evaluating expressions inside {{ }} delimiters, and it allows you to dynamically display values within your templates. For example:
htmlCopyEdit<div>{{ myVariable }}</div>
However, if there is an error evaluating the expression inside the interpolation delimiters, AngularJS throws the [$interpolate:interr] error.
Step 1: Understanding the Cause of the Error
AngularJS throws this error if there is a problem with the expression being interpolated. Some common causes include:
- Invalid Syntax in the Expression: The expression inside
{{ }}may be improperly formed, such as missing or mismatched parentheses, operators, or incomplete expressions. - Undefined Variables or Functions: The variable or function you’re trying to interpolate might not exist or may be undefined in the current scope.
- Circular Dependencies: Sometimes, circular references or dependencies between variables can cause this error because AngularJS struggles to resolve the value.
- Complex Expressions: AngularJS expressions that involve complex logic or unsupported operations might trigger the error.
- Interpolation Inside Directives: If you use interpolation within a directive’s attribute or property value, AngularJS may not evaluate it as expected, especially if the directive is not set up to handle expressions properly.
Step 2: Common Causes of the Error
- Syntax Issues in Interpolated Expressions
The most common issue is a syntax problem in the expression you’re trying to interpolate. For instance, an incomplete or incorrectly formed expression can break AngularJS’ evaluation system.
Example of incorrect syntax:
<div>{{ someVar + }} </div> <!-- Incorrect expression with missing part -->
2. Undefined or Null Variables
If you’re attempting to display or manipulate a variable that hasn’t been defined or initialized in your scope, AngularJS won’t be able to evaluate the expression.
Example:
<div>{{ undefinedVar }}</div> <!-- undefinedVar is not defined in the scope -->
This will cause the interpolation to fail, as AngularJS cannot resolve the value of undefinedVar.
3. Use of Functions or Methods Inside Interpolation
If you’re using functions inside interpolation and they are not available in the current scope, AngularJS cannot evaluate them.
Example:
<div>{{ myFunction() }}</div> <!-- myFunction is undefined or not available in the scope -->
4. Complex Expressions or Logic
AngularJS interpolation doesn’t support complex expressions like loops, conditionals, or function calls directly inside {{ }}. If you try to perform complex logic inside the interpolation, you may encounter this error.
Example of complex expression causing the error:
<div>{{ (x > y ? 'Yes' : 'No') && someVar }}</div> <!-- Complex logic -->
5. Interpolation Inside Directive Attributes
Sometimes, if you’re using AngularJS expressions within custom directives and their attributes, AngularJS might not interpret them as expressions correctly. You might need to ensure that you’re using ng-attr-* or similar approaches for attributes to handle expressions.
Example:
<div ng-class="{{ someClass }}"> </div> <!-- This might cause issues if not properly bound -->
Step 3: How to Fix the [$interpolate:interr] Error
1. Check the Expression Syntax
Ensure that the expressions inside {{ }} are correctly formed, complete, and without syntax errors.
Example:
<div>{{ someVar + 1 }}</div> <!-- Correct expression -->
2. Initialize All Variables Before Use
Ensure that all variables you’re interpolating are initialized and available in the scope before you try to display them.
Example:
$scope.someVar = 5; // Ensure variables are initialized before use
3. Use Safe Navigation for Undefined or Null Variables
If you’re uncertain whether a variable is defined, you can use AngularJS’ safe navigation operator to prevent errors when attempting to access properties of null or undefined variables.
Example:
<div>{{ user.name || 'Default Name' }}</div> <!-- Use a fallback if user.name is undefined -->
Alternatively, you can use AngularJS’ ng-if or ng-show to conditionally render content based on whether the variable is defined.
Example:
<div ng-if="user">{{ user.name }}</div> <!-- Only display if user is defined -->
4. Avoid Complex Expressions in Interpolation
If you need to use complex logic, perform it in the controller or a function and then bind the result to the view instead of trying to handle it directly in the interpolation.
Example:
<div>{{ myFunction() }}</div> <!-- This will not work if myFunction is not available -->
Instead, precompute the value in the controller:
$scope.computedValue = myFunction();
Then, bind it in the template:
<div>{{ computedValue }}</div>
5. Ensure Correct Use of Directives and Expressions
If you’re using interpolation inside directive attributes, use ng-attr-* to bind expressions to HTML attributes.
Example:
<div ng-attr-class="{{ someClass }}"> </div> <!-- Correct usage of ng-attr- for dynamic attributes -->
6. Avoid Function Calls or Complex Logic Directly in Interpolation
Keep logic simple within interpolated expressions. Avoid invoking functions or performing complex operations directly in the {{ }} delimiters.
Instead, handle logic in the controller:
$scope.result = someCalculation();
Then bind the result in the HTML:
<div>{{ result }}</div>
Step 4: Debugging the Error
- Use Console Logs: If the error message is vague, use
console.log()in your controller to inspect the values of variables you are trying to interpolate. This can help you identify missing or undefined values.
Example:
console.log($scope.someVar); // Check the value of the variable
- Check Your Browser’s Developer Console: The browser’s developer tools (e.g., Chrome DevTools) can often provide more detailed information about the error and the specific line of code that triggered it.
- Validate with AngularJS Batarang: Use AngularJS Batarang (a Chrome extension) to inspect the AngularJS scope and ensure that all values you are interpolating are available.
Example Fixes:
Example 1: Missing Variable Definition
Before:
<div>{{ user.name }}</div> <!-- This will fail if user is not defined -->
After:
<div>{{ user.name || 'Default Name' }}</div> <!-- Use fallback for undefined values -->
Example 2: Complex Expression in Interpolation
Before:
<div>{{ (x > y ? 'Yes' : 'No') && someVar }}</div> <!-- Complex logic -->
After:
$scope.computedValue = (x > y ? 'Yes' : 'No') && someVar;
In HTML:
<div>{{ computedValue }}</div>
Step 5: Final Considerations
- AngularJS Expression Limitations: Always remember that AngularJS expressions are evaluated as JavaScript code, but with limitations on the types of expressions. Avoid complex expressions and try to move logic to the controller where possible.
- Check for Circular Dependencies: If you’re using multiple expressions that depend on each other in a circular way, AngularJS might not be able to resolve them. Try to break circular references by reorganizing your logic.