SyntaxError — Parse-Time Failures

TL;DR

Explanation

SyntaxError is thrown at parse time before any code runs. The entire script block fails — not just the broken function. Common causes: missing closing brackets/braces, invalid JSON in JSON.parse() (single quotes, trailing commas), using reserved words as identifiers (class, let, const in non-strict modes), template literal syntax errors, regex syntax errors. JSON.parse() throws SyntaxError for invalid JSON — always wrap in try/catch. In modules, import/export syntax errors prevent the entire module graph from loading.

Common Misconception

Why It Matters

Common Mistakes

• Not wrapping JSON.parse() in try/catch for external data.
• Using single quotes in JSON strings — JSON requires double quotes.
• eval() throwing SyntaxError — another reason to avoid eval.

Code Examples

// JSON with single quotes — SyntaxError:
const data = JSON.parse("{'name': 'Paul'}"); // SyntaxError

// Trailing comma in JSON:
const config = JSON.parse('{"debug": true,}'); // SyntaxError

try {
    const data = JSON.parse(response);
} catch (e) {
    if (e instanceof SyntaxError) {
        console.error('Invalid JSON:', e.message);
    } else {
        throw e; // Re-throw non-syntax errors
    }
}

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_token

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