JavaScript’s not-so-obvious type coercion examples

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)