undefined
In JavaScript, undefined is a primitive type whose only value, also named undefined, stands for a variable, property, or result that hasn’t been given a value. The global name undefined is a read-only property of the global object, not a keyword or a literal, while null is a literal and a reserved word. Both are primitive types:
> let total;
undefined
> total
undefined
> typeof total
'undefined'
Like every primitive, undefined is immutable and compares by value. It’s the type’s only value, so undefined === undefined and Object.is(undefined, undefined) both return true.
Where undefined Appears and typeof
JavaScript produces undefined for a declared variable without an initializer, a missing property, a missing argument, a function call that doesn’t return a value, and any expression passed to the void operator:
> const user = { name: "Ada" };
undefined
> user.email
undefined
> function greet(name) { return name; }
undefined
> greet()
undefined
> function logName() {}
undefined
> logName()
undefined
> void 0
undefined
The typeof operator returns 'undefined' for the value and also for a name that was never declared, where reading the name directly throws a ReferenceError. The exception is a let, const, or class name used before its declaration, where typeof throws a ReferenceError too:
> typeof missingName
'undefined'
> missingName
Uncaught ReferenceError: missingName is not defined
Reading a property of undefined throws a TypeError.
Comparing and Converting undefined
Loose equality treats undefined and null as equal to each other and to nothing else in standard JavaScript, so value == null matches both values, while value === undefined matches only undefined. Other conversions follow the type coercion rules, and undefined is falsy:
> undefined == null
true
> undefined === null
false
> undefined == 0
false
> Number(undefined)
NaN
> String(undefined)
'undefined'
> Boolean(undefined)
false
Common undefined Gotchas
Default parameter values apply when an argument is undefined, either missing or passed explicitly, but not when it’s null:
> function welcome(name = "guest") { return `Hello, ${name}!`; }
undefined
> welcome(undefined)
'Hello, guest!'
> welcome(null)
'Hello, null!'
JSON.stringify() drops object properties whose value is undefined and writes undefined array elements as null:
> JSON.stringify({ name: "Ada", email: undefined })
'{"name":"Ada"}'
> JSON.stringify([1, undefined, 3])
'[1,null,3]'
A property set to undefined and a missing property read the same, so telling them apart takes the in operator, which also finds inherited properties, or Object.hasOwn(), which checks only the object’s own properties:
> const settings = { theme: undefined };
undefined
> settings.theme === settings.fontSize
true
> "theme" in settings
true
> "fontSize" in settings
false
Assigning to the global undefined is silently ignored in sloppy mode and throws a TypeError in strict mode. Because undefined isn’t a reserved word, though, a local variable or parameter can still use undefined as its name and shadow the global value.
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By Leodanis Pozo Ramos • Updated Nov. 4, 2026
