How to Get Type of a Variable in JavaScript

1. Using the typeof Operator to Find the Type of Variable
2. Using the typeof Operator in Conditional Checks
3. Note:

Comparing to other programming languages like C, Java etc., JavaScript gives the liberty to developers to define variables of any type with a single keyword (the var keyword). JavaScript, in turn, decides the data type of the variable, later, depending on the values assigned to these variables. It is seemingly easy to determine the data type of a variable. But some scenarios can put us in a fix. Especially in the case of values returned by the REST API response of the server, we may need to know the type of the value or variable before we further code to process it.

Using the typeof Operator to Find the Type of Variable

typeof is a unary operator in javascript that returns the type of an operand to which it is applied. It usually returns the variable type as a string object. There are standard return types to the typeof operator in javascript.

• string: typeof returns string for a variable type string.
• number: It returns number for a variable holding an integer or a floating-point value.
• boolean: For a variable holding true or false values, typeof returns boolean.
• undefined: In case we do not assign values to a variable, the type of the variable will be marked as undefined by JavaScript. Hence, the typeof operand will return undefined for such undeclared variables.
• object: For variables that hold an array, or an object in {}, or variables assigned with null value, javascript considers the type of such variables as an object. Hence, the typeof operand will return object.
• function: JavaScript allows us to assign functions to a variable. For such cases, the type of such variables will be function. The typeof operator will return function for a function assignment.

The following code snippet demonstrates the behaviour of the typeof operator for different variable assignments and different scenarios.

var s1 = 'hello';
var n1 = 120;
var n1 = 11.1234;
var b1 = true;
var x;
var u = undefined;
var o1 = null;
var o2 = {id: 1, value: 200};
var o3 = [1, 2, 3];
var f = function() {
  return 1 + 2
};

console.log(typeof s1);
console.log(typeof n1);
console.log(typeof n1);
console.log(typeof b1);
console.log(typeof x);
console.log(typeof u);
console.log(typeof o1);
console.log(typeof o2);
console.log(typeof o3);
console.log(typeof f);

Output:

"string"
"number"
"number"
"boolean"
"undefined"
"undefined"
"object"
"object"
"object"
"function"

Using the typeof Operator in Conditional Checks

We can use the typeof operator in conditional-checks like in the if block by checking the value returned by the operator and comparing it with the standard type values. We use the === operator for comparison as it includes type checks of operands at both ends of the operator.

var a = 'hello';
if (typeof a === 'string') {
  console.log(true)
}

Output:

true

Similarly, we can do conditional checks for number, boolean, object and even for function. As a best practice, we should create a constant variable for the standard data types returned by the typeof operator of javascript. Then, compare the typeof of a variable against the declared constants. This approach makes it easy to code and reduces typos while writing the conditional blocks, which are usually not caught in a glance. Refer to the following code to understand better.

const STRING_TYPE = 'string';
const NUMBER_TYPE = 'number';
var a = 'hello';
var b = 123;
if (typeof a === STRING_TYPE) {
  console.log(true)
}
if (typeof b === NUMBER_TYPE) {
  console.log(true)
}

Output:

true
true

Note:

• For cases having a variable assigned with a new keyword, javascript will consider such an assignment as an object. Hence the typeof operator will return object for such assignments. Refer to the following code.

var s = new String('hello');
var n = new Number(100);
console.log(typeof s);
console.log(typeof n);

Output:

object
object

• If we assign a function using the new keyword, then the data type of such variables is taken as a function by javascript. The typeof variable with new function() will return as function instead of object. Let us look at the following code.

var fn = new Function();

Output:

"function"

• All browsers support the typeof operator of javascript, including the old versions of Internet Explorer. Hence we can use the operator without any worries in a project supported on multiple browsers.