JavaScript Unary Operators The typeof operator
Example
The typeof operator returns the data type of the unevaluated operand as a string.
Syntax:
typeof operand
Returns:
These are the possible return values from typeof:
| Type | Return value |
|---|---|
Undefined | "undefined" |
Null | "object" |
Boolean | "boolean" |
Number | "number" |
String | "string" |
Symbol (ES6) | "symbol" |
Function object | "function" |
document.all | "undefined" |
| Host object (provided by the JS environment) | Implementation-dependent |
| Any other object | "object" |
The unusual behavior of document.all with the typeof operator is from its former usage to detect legacy browsers. For more information, see Why is document.all defined but typeof document.all returns "undefined"?
Examples:
// returns 'number'
typeof 3.14;
typeof Infinity;
typeof NaN; // "Not-a-Number" is a "number"
// returns 'string'
typeof "";
typeof "bla";
typeof (typeof 1); // typeof always returns a string
// returns 'boolean'
typeof true;
typeof false;
// returns 'undefined'
typeof undefined;
typeof declaredButUndefinedVariable;
typeof undeclaredVariable;
typeof void 0;
typeof document.all // see above
// returns 'function'
typeof function(){};
typeof class C {};
typeof Math.sin;
// returns 'object'
typeof { /*<...>*/ };
typeof null;
typeof /regex/; // This is also considered an object
typeof [1, 2, 4]; // use Array.isArray or Object.prototype.toString.call.
typeof new Date();
typeof new RegExp();
typeof new Boolean(true); // Don't use!
typeof new Number(1); // Don't use!
typeof new String("abc"); // Don't use!
// returns 'symbol'
typeof Symbol();
typeof Symbol.iterator;
