Syntax error VS runtime error

JavaScriptJunior~2 min read

1. What a syntax error is

A syntax error (SyntaxError) is an error that occurs before the program even starts running, when JavaScript cannot parse the code because the language's syntax rules are violated.

Simply put: the code cannot even begin to run.

Examples of syntax errors

javascript

if (true {          // missing parenthesis
  console.log("ok");
}

javascript

function() {        // a function without a name
  console.log("hi");
}

javascript

const a = ;         // no value after "="

The interpreter detects such errors at the parsing stage (before execution), so the code does not run at all.

An example with an explanation

javascript

console.log("before the error");
eval("let x = ;"); // SyntaxError
console.log("after the error"); // will not run

The error occurs immediately when trying to interpret the string, so the program never reaches the second console.log.

2. What a runtime error is

A runtime error is an error that occurs while the program is running, after the code has already passed syntax checking successfully.

That is, the syntax is correct, but the program's logic leads to a failure during execution.

Examples of runtime errors

javascript

console.log(a); // ReferenceError: a is not defined

javascript

let obj = null;
obj.name; // TypeError: Cannot read properties of null

javascript

JSON.parse("invalid JSON"); // SyntaxError, but raised during execution

In these cases the code runs, but breaks at a specific line during execution.

3. The main difference

Criterion	Syntax error	Runtime error
When it occurs	At the parsing stage (before the code runs)	While the program is running
Can it be caught with `try...catch`	No (the code will not even start)	Yes, it can be handled
Example	`if (true {`	`console.log(a)`
JS reaction	The code does not run at all	Execution stops at the error
Error type	`SyntaxError`	`ReferenceError`, `TypeError`, `RangeError`, etc.

4. An example for a clear comparison

javascript

// Syntax error:
try {
  eval("if (true { console.log('hi') }"); // <-- invalid syntax
} catch (e) {
  console.log("Caught?", e.message);
}
// → this code will not even run, because the error occurs before execution

// Runtime error:
try {
  console.log(a); // ReferenceError
} catch (e) {
  console.log("Caught:", e.message);
}

Summary

Term	What it means
SyntaxError	An error in the code's structure - JS cannot interpret it
Runtime Error	An error while running syntactically correct code
Difference	The first happens at the parsing stage, the second at the execution stage

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