SyntaxError: illegal character

SyntaxError: Invalid or unexpected token (V8-based)
SyntaxError: illegal character U+201C (Firefox)
SyntaxError: Invalid character '\u201c' (Safari)

There is an invalid character that the interpreter doesn't understand. You should either put it in a string literal or replace it with another character. Use an editor that supports syntax highlighting and carefully check your code against mismatches like a minus sign (-) versus a dash (–) or simple quotes (") versus non-standard quotation marks (“).

Some characters look similar, but will cause the parser to fail interpreting your code. Famous examples of this are quotes, the minus or semicolon (greek question mark (U+37e) looks same).

“This looks like a string”; // SyntaxError: illegal character
// “ and ” are not " but look like it

42 – 13; // SyntaxError: illegal character
// – (en-dash) is not - but looks like it

const foo = "bar"; // SyntaxError: illegal character
// <37e> is not ; but looks like it

This should work:

"This is actually a string";
42 - 13;
const foo = "bar";

Some editors and IDEs will notify you or at least use a slightly different highlighting for it, but not all. When something like this happens to your code and you're not able to find the source of the problem, it's often best to just delete the problematic line and retype it.

It's easy to forget a character here or there.

const operators = ["+", "-", ×", "÷"];
// SyntaxError: illegal character U+00D7

Add the missing quote for "×".

const operators = ["+", "-", "×", "÷"];

When copy pasting code from external sources, there might be invalid characters. Watch out!

const foo = "bar";​
// SyntaxError: illegal character

When inspecting this code in an editor like Vim, you can see that there is actually a zero-width space (ZWSP) (U+200B) character.

const foo = "bar";<200b>

• Lexical grammar