JavaScript Type System
The
typeofoperator reports what kind of value you are holding.
It returns a string naming the type.
It has two famous quirks that are worth learning early.
Example
javascript
console.log(typeof "hello");
console.log(typeof 42);
console.log(typeof true);The output is string, number, boolean.
One of: string, number, boolean, undefined, object, function, symbol or bigint.
It always returns a string, never the type itself.
It is safe to use on a variable that was never declared.
Syntax
javascript
typeof valueNo brackets are needed, though they are allowed.
typeof null is object, which is a bug from the very first version of JavaScript.
It was never fixed because too much code depends on it.
Example
javascript
console.log(typeof null);
console.log(null === null);To check for null, compare against it directly.
typeof cannot tell an array from an object.
Array.isArray is the right tool.
Example
javascript
console.log(typeof []);
console.log(Array.isArray([]));
console.log(Array.isArray({}));The output is object, true, false.
Functions are objects, but typeof gives them their own answer.
Example
javascript
function greet() {}
console.log(typeof greet);
console.log(typeof class Thing {});A class reports as function, because that is what it is underneath.
Reading an undeclared variable throws, but typeof does not.
This makes it useful for feature checks.
Example
javascript
console.log(typeof somethingNeverDeclared);The output is undefined, with no error.
This is the everyday use: make sure something is what you expect.
Example
javascript
function shout(value) {
if (typeof value !== "string") {
return "not a string";
}
return value.toUpperCase();
}
console.log(shout("hi"));
console.log(shout(42));The guard stops the method call failing on a number.
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript typeof</title>
</head>
<body>
<h1>typeof</h1>
<p id="out"></p>
<script>
const values = ["hello", 42, true, undefined, null, [], {}, function () {}];
const lines = values.map(function (value) {
return JSON.stringify(value === undefined ? "undefined" : value) +
" is " + typeof value;
});
document.getElementById("out").innerHTML = lines.join("<br>");
</script>
</body>
</html>Run the above example in the Try It Editor.
Try adding a number as text:
Add "42" to the list and see it report string, not number.
typeof returns a string naming the type.typeof null is object, which is a long-standing bug.Array.isArray.typeof is the first tool for finding out what you are holding.
Its two quirks - null and arrays - catch out almost everyone once.
Knowing them makes type checking straightforward.