Methods to Check if a Value is a Number in TypeScript

typescript how to check if a value is a number?

In TypeScript, you can check if a value is a number using several methods. Here are some common approaches:

1. Using typeof Operator: The typeof operator can be used to check if a value is of type 'number'. This works for both integer and floating-point numbers.

   function isNumber(value: any): boolean {
       return typeof value === 'number';
   }
   
   console.log(isNumber(123)); // true
   console.log(isNumber('123')); // false
   console.log(isNumber(NaN)); // true
   

2. Using Number.isFinite Method: The Number.isFinite method can be used to check if a value is a finite number. This method is more robust as it filters out NaN, Infinity, and -Infinity.

   function isFiniteNumber(value: any): boolean {
       return typeof value === 'number' && Number.isFinite(value);
   }
   
   console.log(isFiniteNumber(123)); // true
   console.log(isFiniteNumber('123')); // false
   console.log(isFiniteNumber(NaN)); // false
   console.log(isFiniteNumber(Infinity)); // false
   

3. Using Number Constructor and isNaN Function: You can use the Number constructor to attempt to convert a value to a number and then use isNaN to check if the result is NaN.

   function isNumber(value: any): boolean {
       return !isNaN(Number(value));
   }
   
   console.log(isNumber(123)); // true
   console.log(isNumber('123')); // true
   console.log(isNumber('abc')); // false
   console.log(isNumber(NaN)); // false
   

4. Using parseFloat or parseInt: You can use parseFloat or parseInt to check if a value can be parsed as a number. However, this method is less strict and may produce unexpected results for certain inputs.

   function isNumber(value: any): boolean {
       return !isNaN(parseFloat(value)) && isFinite(value);
   }
   
   console.log(isNumber(123)); // true
   console.log(isNumber('123')); // true
   console.log(isNumber('123abc')); // true
   console.log(isNumber('abc')); // false
   

5. Using a Custom Type Guard: You can create a custom type guard to check if a value is a number. This is useful if you want to narrow the type within a TypeScript type system.

   function isNumber(value: any): value is number {
       return typeof value === 'number' && !isNaN(value);
   }
   
   let value: any = 123;
   if (isNumber(value)) {
       // TypeScript now knows that 'value' is a number
       console.log(value.toFixed(2)); // 123.00
   }
   

Choose the method that best fits your use case. For most scenarios, using typeof combined with Number.isFinite is a robust and straightforward approach.