How to check if a string is a number JavaScript
Understanding the data types we work with is crucial when building applications. In JavaScript, there are eight data types, and knowing how to handle them is essential.
JavaScript provides a variety of syntaxes that enable us to perform different operations on these data types.
In this Answer we will focus on checking if a string is a number. This knowledge is valuable for different operations.
Let’s get to it!
Check if a string is a number
There are various methods to check if a string is a number in javascript. We’ll go over the most commonly used and simplest methods.
- By using the
typeofOperator - By using the regex
1. Using the typeof operator
The typeof operator returns the data type of any variable passed into it. We can use it with isNaN to check if a string is a number.
Note:
isNaNchecks if a value is not a number. It returnstrueif the value is not, andfalseif it’s a number.
Here’s an example leveraging its operation for this use case:
let string1 = "Akande";let string2 = "257";function checkString(string){if(typeof string === "string"){console.log(!isNaN(string));}}checkString(string1);checkString(string2);
Explanation
-
Line 1–2: We assign two strings - one is a number and the other a string - to two variables that store them.
-
Line 4: We create a function that takes in an argument. This function contains a block of code that checks if a string is a number
-
Line 5: Contains an
if elsestatement. Theifstatement contains the code that checks if a string is a number.
Note: typeof string === “string” implies that if the type of the
stringis a string, it should run the code inside theifstatement.
- Line 6: Contains the actual code.
!isNaNis used to check if the value ofstringis a number. But it is flipped i.e. It will work in the opposite direction.
Note: Here, if the value is not a number, it returns
false, and if the value is a number,true.
Overall, if string is a number, it returns true. If it isn’t, it returns false.
2. Using regex
Regex or regular expressions are patterns that help programmers match, search, and replace texts. The regex used to test for numbers is ^[0-9]*＄. We can use this to check if a string is a number with .test() regex method.
let string1 = "Akande";let string2 = "257";function checkString(string) {return /^[0-9]*$/.test(string);}console.log(checkString(string1));console.log(checkString(string2));
Explanation
- Line 1–2: We assign two strings - one is a number and the other a string - to two variables that store them.
-
Line 4: We create a function that takes in an argument. This function contains a block of code that checks if a string is a number.
-
Line 5: We use
.test()to test the string against the regex^[0-9]*＄, which tests for numbers. Ifstringpasses the test, it returnstrue. A failure will returnfalse.
In other words, if string is a number, it returns true, and if not a number; it returns false.
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