AUR0151

Error Message

AUR0151: Expression error: invalid start: "{{expression}}"

Where {{expression}} is the problematic binding expression.

Description

This error occurs when a binding expression starts with an invalid token or character that the expression parser cannot process. The Aurelia expression parser expects expressions to begin with valid JavaScript-like syntax.

Common Scenarios

Invalid Starting Characters

<!-- ❌ Wrong: Starting with invalid characters -->
<div textcontent.bind="@invalid"></div>
<div textcontent.bind="#invalid"></div>
<div textcontent.bind="$invalid"></div>

Malformed Property Access

<!-- ❌ Wrong: Starting with dot -->
<div textcontent.bind=".property"></div>

<!-- ❌ Wrong: Starting with bracket -->
<div textcontent.bind="[0]"></div>

Invalid Operators at Start

<!-- ❌ Wrong: Starting with operators -->
<div textcontent.bind="+ value"></div>
<div textcontent.bind="* multiplier"></div>
<div textcontent.bind="&& condition"></div>

Solutions

1. Fix Property Access

<!-- ✅ Correct: Proper property access -->
<div textcontent.bind="item.property"></div>
<div textcontent.bind="items[0]"></div>
<div textcontent.bind="this.property"></div>

2. Use Valid Variable Names

<!-- ✅ Correct: Valid identifiers -->
<div textcontent.bind="userName"></div>
<div textcontent.bind="_privateValue"></div>
<div textcontent.bind="$specialValue"></div> <!-- $ is valid at start -->

3. Proper Expression Syntax

<!-- ✅ Correct: Valid expressions -->
<div textcontent.bind="value + 1"></div>
<div textcontent.bind="condition && result"></div>
<div textcontent.bind="(expression)"></div>

4. Handle Special Cases

<!-- ✅ Correct: Literal values -->
<div textcontent.bind="'string literal'"></div>
<div textcontent.bind="42"></div>
<div textcontent.bind="true"></div>

<!-- ✅ Correct: Method calls -->
<div textcontent.bind="getValue()"></div>
<div textcontent.bind="item.toString()"></div>

Example: Common Fixes

<!-- Template with various expression errors -->
<template>
  <!-- ❌ Wrong expressions -->
  <div textcontent.bind="@user"></div>
  <div textcontent.bind=".name"></div>
  <div textcontent.bind="+ 5"></div>
  
  <!-- ✅ Fixed expressions -->
  <div textcontent.bind="user"></div>
  <div textcontent.bind="user.name"></div>
  <div textcontent.bind="value + 5"></div>
  
  <!-- ✅ Alternative approaches -->
  <div textcontent.bind="getUser()"></div>
  <div textcontent.bind="this.name"></div>
  <div textcontent.bind="5"></div>
</template>

export class MyComponent {
  user = { name: 'John' };
  value = 10;
  
  getUser() {
    return this.user;
  }
}

Debugging Tips

1. Check Expression Syntax: Ensure expressions follow JavaScript syntax rules

2. Validate Property Access: Use proper dot notation or bracket notation

3. Check for Typos: Look for accidental special characters

4. Use Browser DevTools: Inspect the element to see the exact binding expression

5. Test in Console: Try evaluating similar expressions in browser console

Related Errors

• AUR0153 - Expected identifier

• AUR0168 - Unexpected character

• AUR0155 - Unexpected end of expression