JavaScript’s not-so-obvious type coercion examples

Saboor Bhutta — Mon, 02 Jun 2025 05:55:20 +0000

JavaScript’s type coercion and dynamic typing system are full of subtle (and sometimes surprising) behaviors. Understanding these quirks can make you much more confident and effective as a JS developer.

Here’s a list of not-so-obvious JavaScript type coercion examples, grouped into categories with explanations:

1. String vs Number Comparison

Expression	Value	Reason
'10' < '2'	true	Lexicographic comparison: '1' comes before '2'
'10' < 2	false	string '10' is coerced to number 10
'5' == 5	true	string '5' is coerced to number 5
'5' === 5	false	strict equality, no type coercion

Rule: Loose equality (==) allows type coercion. Strict equality (===) does not.

2. Falsy Values

JavaScript has falsy values that act like false in boolean contexts:

Expression	Value
Boolean(' ')	false
Boolean(0)	false
Boolean(null)	false
Boolean(undefined)	false
Boolean(NaN)	false
Boolean([])	true
Boolean({})	true

Note: [] and {} are truthy, even though they're empty.

3. Array and Object Coercion

Expression	Value	Reason
[] + []	' '	both arrays are coerced to strings → ' ' + ' '
[] + {}	'[object Object]'	[] → ' ', {} → '[object Object]' ' ' + '[object Object]' → '[object Object]'
[] == false	true	[] → ' ' → ' ' == false → true
[0] == false	true	[0] → '0' → 0 == false → true
[1,2] + [3,4]	'1,23,4'	arrays are stringified → '1,2' + '3,4'

4. Arithmetic with Non-numbers

Expression	Value	Reason
'5' - 2	3	'5' is coerced to number
'5' + 2	'52'	'+' triggers string concatenation
null + 1	1	null → 0
undefined + 1	NaN	undefined → NaN
true + 1	2	true → 1
false + 1	1	false → 0
"abc" * 33	NaN	Cannot coerce "abc" to Number → NaN
33 / 0	Infinity	Division by zero → Infinity
0 / 0	NaN	Undefined division → NaN

Note: The + operator behaves differently depending on whether one side is a string — it prefers string concatenation over numeric addition.

5. Equality Weirdness

Expression	Value	Reason
null == undefined	true	only loosely equal to each other
null === undefined	false	different types
[] == ' '	true	[] → ' '
[null] == ' '	true	[null] → ' '
[undefined] == ' '	true	[undefined] → ' '
{} == {} OR [] == []	false	different references
NaN == NaN	false	NaN is not equal to anything, even itself

6. `typeof` Quirks

Expression	Value	Reason
typeof null	'object'	Legacy bug in JavaScript
typeof NaN	'number'	NaN is actually a special kind of number
typeof [] , typeof {}	'object'	Arrays are objects too
typeof function(){}	'function'	Functions are a special kind of object

Best Practices

Always use === instead of == unless you're very sure about coercion.
Use Number(), Boolean(), or String() explicitly when converting types.
For checking arrays: Array.isArray(value)
For checking NaN: Number.isNaN(value) or Object.is(value, NaN)

DEV Community: Saboor Bhutta