How do you check the data type of a variable?
TL;DR
To check the data type of a variable in JavaScript, you can use the typeof
operator. For example, typeof variableName
will return a string indicating the type of the variable, such as "string"
, "number"
, "boolean"
, "object"
, "function"
, "undefined"
, or "symbol"
. For arrays and null
, you can use Array.isArray(variableName)
and variableName === null
, respectively.
How do you check the data type of a variable?
Using the typeof
operator
The typeof
operator is the most common way to check the data type of a variable in JavaScript. It returns a string indicating the type of the operand.
let str = 'Hello, world!';
console.log(typeof str); // "string"
let num = 42;
console.log(typeof num); // "number"
let bool = true;
console.log(typeof bool); // "boolean"
let obj = { name: 'Alice' };
console.log(typeof obj); // "object"
let func = function () {};
console.log(typeof func); // "function"
let undef;
console.log(typeof undef); // "undefined"
let sym = Symbol();
console.log(typeof sym); // "symbol"
Checking for null
The typeof
operator returns "object"
for null
, which can be misleading. To specifically check for null
, you should use a strict equality comparison.
let n = null;
console.log(n === null); // true
Checking for arrays
Arrays are a special type of object in JavaScript. To check if a variable is an array, you can use the Array.isArray
method.
let arr = [1, 2, 3];
console.log(Array.isArray(arr)); // true
Checking for NaN
NaN
(Not-a-Number) is a special numeric value in JavaScript. To check if a value is NaN
, you can use the Number.isNaN
method.
let notANumber = NaN;
console.log(Number.isNaN(notANumber)); // true
Checking for null
and undefined
To check if a variable is either null
or undefined
, you can use a combination of the ==
operator and the typeof
operator.
let value = null;
console.log(value == null); // true
let value2;
console.log(typeof value2 === 'undefined'); // true