🌋 JavaScript Error and Warning Reference

When I just started my coding journey, errors felt like my greatest enemy. I was always intimidated by them — every red line on the screen felt like a personal failure, and I often wondered if I was cut out for programming at all. But over time, I realized that errors aren’t enemies; they are my greatest teachers. Every mistake revealed a new lesson, every bug pushed me to think deeper, and every warning guided me toward writing better code.

To learn from errors, you first have to recognize them. This cheatsheet will help you do exactly that.

---

JavaScript Errors

Error	Description	Example
AggregateError: No Promise in Promise.any was resolved	All promises in Promise.any() were rejected, so no promise resolved.	Promise.any([Promise.reject("fail")])
Error: Permission denied to access property “x”	Accessing a restricted or cross-origin property causes this error.	window.parent.location.href (from different origin iframe)
InternalError: too much recursion	Call stack exceeded due to infinite or very deep recursion.	function recurse() { recurse(); } recurse();
RangeError: argument is not a valid code point	Unicode code point is out of valid range (0 to 0x10FFFF).	String.fromCodePoint(0x110000)
RangeError: BigInt division by zero	Dividing a BigInt by zero (0n) is not allowed.	10n / 0n
RangeError: BigInt negative exponent	BigInt exponentiation does not support negative exponents.	2n ** -1n
RangeError: form must be one of ‘NFC’, ‘NFD’, ‘NFKC’, or ‘NFKD’	Invalid Unicode normalization form string.	'abc'.normalize('XYZ')
RangeError: invalid array length	Array length must be a non-negative integer less than 2^32.	new Array(-1)
RangeError: invalid date	Date constructor received invalid date string or value.	new Date('invalid')
RangeError: precision is out of range	Precision value outside allowed range in toPrecision().	(1.23).toPrecision(500)
RangeError: radix must be an integer	Radix for parsing must be an integer between 2 and 36.	parseInt('10', 2.5)
RangeError: repeat count must be less than infinity	Repeat count cannot be infinite or too large.	'a'.repeat(Infinity)
RangeError: repeat count mus t be non-negative	Repeat count cannot be negative.	'a'.repeat(-1)
RangeError: x can’t be converted to BigInt because it isn’t an integer	Only integer values can be converted to BigInt.	BigInt(1.5)
ReferenceError: “x” is not defined	Variable used before being declared or not declared at all.	console.log(foo);
ReferenceError: assignment to undeclared variable “x”	Assigning to a variable not declared in strict mode.	'use strict'; foo = 123;
ReferenceError: can’t access lexical declaration ‘X’ before initialization	Accessing a let or const variable before declaration.	console.log(x); let x = 5;
ReferenceError: must call super constructor before using ‘this’ in derived class constructor	this used before super() call in subclass constructor.	constructor() { this.x = 1; super(); }
ReferenceError: super() called twice in derived class constructor	Calling super() more than once in a derived class constructor.	constructor() { super(); super(); }
SyntaxError: ‘arguments’/’eval’ can’t be defined or assigned to in strict mode code	Cannot use or assign arguments or eval as identifiers in strict mode.	'use strict'; let arguments = 5;
SyntaxError: “0”-prefixed octal literals are deprecated	Octal literals starting with 0 are deprecated and disallowed in strict mode.	let x = 071;
SyntaxError: “use strict” not allowed in function with non-simple parameters	Strict mode directive disallowed with complex function parameters.	function f(a = 1) { "use strict"; }
SyntaxError: “x” is a reserved identifier	Using reserved words or identifiers as variable/function names.	let for = 5;
SyntaxError: \ at end of pattern	Regex pattern ends with unescaped backslash.	/abc\\/
SyntaxError: a declaration in the head of a for-of loop can’t have an initializer	let or const in loop header cannot be initialized.	for (let i = 0 of arr) {}
SyntaxError: applying the ‘delete’ operator to an unqualified name is deprecated	Using delete on a variable name is deprecated.	delete x;
SyntaxError: arguments is not valid in fields	Using arguments as class field name is invalid.	class C { arguments = 1; }
SyntaxError: await is only valid in async functions, async generators and modules	await used outside async function or module.	await fetch('url');
SyntaxError: await/yield expression can’t be used in parameter	Using await or yield in function parameter list is invalid.	function f(await x) {}
SyntaxError: cannot use ?? unparenthesized within \|\| and && expressions	Nullish coalescing operator requires parentheses when combined with || or &&.	true || false ?? true
SyntaxError: character class escape cannot be used in class range in regular expression	Using escape sequences improperly inside regex character ranges.	/[a-\d]/
SyntaxError: continue must be inside loop	continue statement used outside loops.	if (true) { continue; }
SyntaxError: duplicate capture group name in regular expression	Regex contains named groups with duplicate names.	/(?'name'a)(?'name'b)/
SyntaxError: duplicate formal argument x	Function parameters declared more than once.	function f(x, x) {}
SyntaxError: for-in loop head declarations may not have initializers	Loop variable declaration cannot have initializer.	for (let i = 0 in obj) {}
SyntaxError: function statement requires a name	Named function missing a name.	function () {}
SyntaxError: functions cannot be labelled	Function declarations cannot have labels.	label: function foo() {}
SyntaxError: getter and setter for private name #x should either be both static or non-static	Private getter and setter must both be static or non-static.	class C { static get #x() {}; set #x(v) {} }
SyntaxError: getter functions must have no arguments	Getters cannot have parameters.	get x(a) {}
SyntaxError: identifier starts immediately after numeric literal	No space between number and identifier.	let x = 5y;
SyntaxError: illegal character	Invalid character in source code.	var x = "\u0000";
SyntaxError: import declarations may only appear at top level of a module	import used inside blocks or scripts not marked as modules.	if (true) { import x from 'mod'; }
SyntaxError: incomplete quantifier in regular expression	Quantifier in regex missing parts.	/a{2,}/ (if incomplete)
SyntaxError: invalid assignment left-hand side	Left side of assignment operator invalid.	5 = x;
SyntaxError: invalid BigInt syntax	Invalid format for BigInt literal.	123n4
SyntaxError: invalid capture group name in regular expression	Regex named capture group has invalid name.	/(?<1name>a)/
SyntaxError: invalid character in class in regular expression	Invalid characters inside regex character class.	/[a-]/
SyntaxError: invalid class set operation in regular expression	Invalid use of set operations like subtraction in regex classes.	/[a-z&&[^aeiou]]/ (unsupported)
SyntaxError: invalid decimal escape in regular expression	Invalid decimal escapes in regex.	/\9/
SyntaxError: invalid identity escape in regular expression	Invalid escape sequences in regex.	/\c/
SyntaxError: invalid named capture reference in regular expression	Reference to undefined named group in regex.	/\k<name>/ without group named name
SyntaxError: invalid property name in regular expression	Invalid property names in Unicode regex escapes.	/\p{Invalid}/u
SyntaxError: invalid range in character class	Character class ranges out of order or invalid.	/[z-a]/
SyntaxError: invalid regexp group	Malformed group in regex.	/(()/
SyntaxError: invalid regular expression flag “x”	Unknown or unsupported regex flags used.	/a/x
SyntaxError: invalid unicode escape in regular expression	Unicode escape sequences are invalid or malformed.	/\u{110000}/u
SyntaxError: JSON.parse: bad parsing	Invalid JSON string passed to JSON.parse().	JSON.parse("{")
SyntaxError: label not found	A break or continue statement references a non-existent label.	break missingLabel;
SyntaxError: missing : after property id	Object literal missing colon between key and value.	let obj = {a 1}
SyntaxError: missing ) after argument list	Function call missing closing parenthesis.	foo(bar;
SyntaxError: missing ) after condition	Control statement condition missing closing parenthesis.	if (x > 0 {}
SyntaxError: missing ] after element list	Array literal missing closing bracket.	let arr = [1, 2, 3;
SyntaxError: missing } after function body	Function body missing closing brace.	function foo() {
SyntaxError: missing } after property list	Object literal missing closing brace.	let obj = { a: 1, b: 2
SyntaxError: missing = in const declaration	const declaration missing initializer.	const x;
SyntaxError: missing formal parameter	Function parameter missing name.	function f(,) {}
SyntaxError: missing name after . operator	Member access missing property name.	obj..prop
SyntaxError: missing variable name	Variable declaration missing identifier.	let ;
SyntaxError: negated character class with strings in regular expression	Negated character class contains string instead of characters.	/[^abc]+/ with invalid usage
SyntaxError: new keyword cannot be used with an optional chain	Cannot use new with optional chaining operator ?..	new obj?.method();
SyntaxError: nothing to repeat	Regex quantifier applied to nothing.	/+/
SyntaxError: numbers out of order in {} quantifier	Quantifier range with min greater than max.	/a{5,2}/
SyntaxError: object destructuring requires an assignment target	Destructuring must be part of assignment or declaration.	({a, b}); alone
SyntaxError: operator expected	Operator missing in expression.	x 5;
SyntaxError: parameters list has duplicate element	Function parameters duplicated.	function f(x, x) {}
SyntaxError: private field ‘#x’ must be declared in an enclosing class	Private class field used without declaration in class.	this.#x without #x declaration
SyntaxError: private field ‘#x’ must be declared in an enclosing class	Private field access on wrong class or without declaration.	other.#x
SyntaxError: property or method expected	Missing property or method name.	class C { get () {} }
SyntaxError: redeclaration of formal parameter	Function parameter redeclared.	function f(a, a) {}
SyntaxError: redeclaration of variable	Variable declared twice in the same scope.	let x; let x;
SyntaxError: return outside function	return used outside function body.	return 5; at top-level code
SyntaxError: script or module missing semicolon before function declaration	Semicolon missing before function declaration causes parse error.	let a = 1 function f() {}
SyntaxError: strict mode code may not include a with statement	with is disallowed in strict mode.	'use strict'; with(obj) {}
SyntaxError: super() must be called before accessing ‘this’ in constructor	Calling this before super() in subclass constructor.	constructor() { this.x; super(); }
SyntaxError: this is not allowed before super() call	Same as above, referencing this before super().	constructor() { this.x = 1; super(); }
SyntaxError: Unexpected identifier	Unexpected name/token in code syntax.	let x = var;
SyntaxError: Unexpected number	Unexpected number found in code.	let x = 5 10;
SyntaxError: Unexpected string	Unexpected string literal in code.	let x = "hello" "world";
SyntaxError: Unexpected template string	Template string used incorrectly.	let x = `hello` `world`;
SyntaxError: Unexpected reserved word	Reserved word used as identifier.	let new = 5;
SyntaxError: Unexpected token	Unexpected character/token in code.	let x = ;
SyntaxError: Unexpected token ILLEGAL	Illegal character in code.	let x = "\u0000";
SyntaxError: Unexpected token, expected “]”	Closing bracket ] expected but not found.	let arr = [1, 2;
SyntaxError: Unexpected token, expected “)”	Closing parenthesis ) expected but missing.	foo(bar;
SyntaxError: Unexpected token, expected “:” in object literal	Colon missing in object literal key-value pair.	let obj = {a 1}
SyntaxError: Unexpected token, expected “;”	Semicolon missing or unexpected token found.	let x = 5 let y = 10;
SyntaxError: Unexpected Unicode escape	Invalid Unicode escape sequence.	let x = "\u{XYZ}";
TypeError: x is not a function	Trying to call a non-function value.	let x = 5; x();
TypeError: Cannot read properties of undefined/null	Accessing properties or methods of undefined or null.	let x; x.y;
TypeError: Assignment to constant variable	Attempting to reassign a constant variable declared with const.	const x = 5; x = 10;
TypeError: Invalid assignment to const ‘x’	Another form of reassignment error for const.	const x = 3; x = 4;
TypeError: ‘super’ keyword unexpected here	Using super outside class constructor or subclass.	super();
URIError: URI malformed	Malformed URI string passed to URI functions.	decodeURIComponent('%')

---

JavaScript Warnings (Not always thrown as errors)

Warning	Description	Example
Duplicate parameter names	Using the same parameter name more than once in a function declaration.	function f(x, x) {}
Implicit global variable assignment	Assigning to undeclared variables creates global variables (in non-strict mode).	foo = 10;
Deprecated feature usage	Using outdated or deprecated features (e.g. escape()).	escape("Hello")
Octal literals in strict mode	Using legacy octal literals (e.g. 071) is disallowed in strict mode.	'use strict'; let x = 071;
Using with statement	with is disallowed in strict mode due to its confusing scoping.	'use strict'; with(obj) {}
Non-strict sloppy declarations	Variables declared without var, let, or const in non-strict mode.	x = 1;
Using deprecated __proto__	Directly accessing or assigning __proto__ is discouraged.	obj.__proto__ = null;

---