The typeof operator

The typeof operator returns the data type of the un-evaluated 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 http://stackoverflow.com/q/40643613/6388243

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;

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