Common Error Messages

Here are some common error messages you may encounter when using complex template expressions:

“Template syntax error: Expected quoted attribute value” — Complex expression in attribute without quotes
“Template syntax error: Unexpected token” — HTML non-compliant character (like <) in text node, or malformed expression
“‘this’ is not allowed in template expressions” — Using this keyword
“‘new’ operator is not allowed in template expressions” — Using new operator
“Function declarations are not allowed in template expressions” — Using function declaration instead of arrow function
“Arrow functions with block bodies are not allowed” — Using block body in arrow function
“Assignment operators are not allowed outside arrow functions” — Assignment outside arrow function
“Update operators are not allowed outside arrow functions” — Update operator (++, --) outside arrow function

HTML Compliance Errors in Text Nodes

Using HTML non-compliant characters like < in text node expressions will cause parsing errors.

1<!-- ❌ Error: < character in text node -->
2<div>{age < 18 ? 'Minor' : 'Adult'}</div>
3<!-- Error: Template syntax error: Unexpected token -->
4
5<!-- ❌ Error: <= operator in text node -->
6<div>{count <= 10 ? 'Low' : 'High'}</div>
7<!-- Error: Template syntax error: Unexpected token -->
8
9<!-- ✅ Valid - use HTML-compliant alternatives -->
10<div>{age >= 18 ? 'Adult' : 'Minor'}</div>
11<div>{count > 10 ? 'High' : 'Low'}</div>

Syntax Errors in Expressions

Malformed expressions will result in syntax errors.

1<!-- ❌ Error: Missing closing parenthesis -->
2<div>{formatCurrency(price}</div>
3<!-- Error: Expected closing parenthesis -->
4
5<!-- ❌ Error: Missing closing brace in template literal -->
6<div>{`Hello ${name`}</div>
7<!-- Error: Expected closing brace in template literal -->
8
9<!-- ❌ Error: Invalid operator usage -->
10<div>{foo && && bar}</div>
11<!-- Error: Unexpected token -->
12
13<!-- ✅ Valid expressions -->
14<div>{formatCurrency(price)}</div>
15<div>{`Hello ${name}`}</div>
16<div>{foo && bar}</div>

1import { LightningElement } from "lwc";
2
3export default class SyntaxErrorsComponent extends LightningElement {
4  price = 99.99;
5  name = "World";
6  foo = true;
7  bar = "value";
8
9  formatCurrency(price) {
10    return `$${price.toFixed(2)}`;
11  }
12}