Javascript check if not a number

01 May 2021 - Long Ly

There are multiple ways to check if a variable is a number in JavaScript and here are some common ways.

• isNaN( variable ) : This function name stands for “is Not a Number”. If the variable is a number, it will return FALSE! Otherwise, it would be TRUE.

• typeof( operand ) : The typeof operator returns a string indicating the type of the unevaluated operand

• match( regex ) : The match() method retrieves the result of matching a string against a regular expression

The function isNaN() is a better way, be used in most cases.
But typeof() and match() are also nice to try.

Becareful when using typeof() because it will return “string” if the number is declared as a string! e.g. let number = “numberA”

While match() is only available for variables which declared in string type.

    let number = 100;
    let isNumber = isNaN(number);
    console.log("scenario 1:", isNumber);
    // expected: false

    let numberInStringType = "100";
    isNumber = isNaN(numberInStringType);
    console.log("scenario 2:", isNumber);
    // expected: false

    number = 100;
    let type = typeof number;
    console.log("scenario 3:", type);
    // expected: "number"

    numberInStringType = "100";
    type = typeof numberInStringType;
    console.log("scenario 4 (failure):", type);
    // expected: "number" but received "string" 
    // => To fix this issue, we can use regular expression to check if the string is a number!  
    // regex to match a number (in string type)
    let matches = numberInStringType.match(/^\d+$/g);
    isNumber = matches.length > 0;
    console.log("scenario 4 (fixed):", isNumber);
    // expected: true