JavaScript: typeof operator

JavaScript Type Checking

The typeof operator is used to get the data type (returns a string) of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. The operator returns the data type.

Syntax

typeof operand
or
typeof (operand)

There are six possible values that typeof returns: object, boolean, function, number, string, and undefined. The following table summarizes possible values returned by the typeof operator.

Examples of typeof operator: string

Console

> typeof ""
"string"
> typeof "abc"
"string"
> typeof (typeof 1)
"string"

Examples of typeof operator: number

Console

> typeof 17
"number"
> typeof -14.56
"number"
> typeof 4E-3
"number"
> typeof Infinity
"number"
> typeof Math.LN2
"number"
> typeof NaN
"number"

Examples of typeof operator: boolean

Console

> typeof false
"boolean"
> typeof true
"boolean"

Examples of typeof operator: function

Console

> typeof Math.tan
"function"
>typeof function(){}
"function"

Examples of typeof operator: object

Console

> typeof {a:1}
"object"
> typeof new Date()
"object"
> typeof null
"object"
> typeof /a-z/
"object"
> typeof Math
"object"
> typeof JSON
"object"

Examples of typeof operator: undefined

Console

> typeof undefined
"undefined"
> typeof abc
"undefined"

More examples on typeof operator

typeof(4+7);  //returns number
typeof("4"+"7"); //returns string
typeof(4*"7"); //returns number
typeof(4+"7"); //returns string

What is the difference between typeof myvar and typeof(myvar) in JavaScript?

There is absolutely no difference between typeof myvar and typeof(myvar). The output of the following codes will be same i.e. "undefined".

alert(typeof myvar);

alert(typeof(myvar));

How to detect an undefined object property in JavaScript?

The following method is the best way to detect an undefined object property in JavaScript.

if (typeof xyz === "undefined")
     alert("xyz is undefined");

How to check whether a variable is defined in JavaScript or not?

The following method is the best way to detect an undefined object property in JavaScript.

if (typeof xyz === "undefined")
     alert("xyz is undefined");
if (typeof xyz === "undefined")
  alert("Varaible is undefined");
else
  alert("Variable is defined"); 

The following example tests the data type of variables.

JS Code

var index = 8;
  var result = (typeof index === 'number');
  alert(result);
  // Output: true
var description = "w3resource";
  var result = (typeof description === 'string');
  alert(result); 
  // Output: true

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