Checking if string is numeric in Typescript and Javascript

I came across a problem of converting a char value to a number when I was working with Javascript. The interesting thing was that the solution just wasn’t as obvious as I initially thought. In this post, I will talk about how I solved the problem and why did I come across it in the first place.

Background

In a previous post, I mentioned how I was solving a bunch of recursive problems on CodingBat.com! Well…I had a bit too much fun with it and I decided to extend the fun by solving string problems as well. Solving the string problems was good easy fun, until I came across a minor issue.

Issue

The problem I was trying to solve was sumDigits. It’s a simple problem i.e.

As I mentioned in my previous post and as you can see, CodingBat is focused on Java. However, the problem description is fairly generic and can be solved in any language.

When writing the code for this problem the first thing that I thought of was Math.abs. For anyone who’s worked with Javascript would know this method. Unfortunately it’s been a while since I solved something like this in Javascript, so the first line of code I wrote for it didn’t work. How?

Solution

Here’s a small snippet of what I was trying to do,

if(Math.abs(str.charAt(0)) != NaN) { }

That clearly didn’t work. Initially, I was trying to solve this in Typescript and I didn’t know of the Number.parseInt method introduced in ES6. As soon as I got to know about that method then it was super easy. Here’s my solution in Typescript.

sumDigits(str: string): number {
    if(str.length == 0) {
        return 0;
    }
    var sum = 0;
    let charArray = str.split("");
    charArray.forEach((val) => {
        let num = parseInt(val);
        if(!isNaN(num)) {
            sum += num;
        }
    });
    return sum;
}

The solution should be fairly self-explanatory but let me describe it. The problem is, we are trying to find out the numeric values in a string and return a sum of all of them. So, we call the split method on the string and a split method with an empty string will return an array of characters. Then again, we use an ES6 method to traverse the array i.e. forEach and if the character is a number we add it to our sum. I think, the only things typescript specific in the above function is the type (i.e. string and number) information. The rest of the code is just plain ES6 Javascript.

p.s. Having types in Javascript is so awesome, thank you MS for Typescript 🙂

Any other way to solve this?

Yes of course. Hence let’s look at a slightly more old-fashioned way of solving this and this time we will solve it in pure Javascript.

CBatSolutions.prototype.sumDigits = function(str) {
    if(str.length == 0) {
        return 0;
    }
    var sum = 0;
    let characterArr = str.split("");
    for(var i=0; i < characterArr.length; i++) {
        var absVal = Math.abs(characterArr[i]);
        if(!Number.isNaN(absVal)) {
            sum += absVal;
        }
    }
    return sum;
}

The difference in this solution is we are using an old for loop where we iterate through every character in an array. (besides being in Javascript and not Typescript).

Any other solution?

Well, yes of course. Why not? We can solve this entire problem recursively. The recursive implementation is actually quite straight forward and clean looking.

CBatSolutions.prototype.sumDigitsR = function(str) {
    if(str.length == 0) {
        return 0;
    }
    var absVal = Math.abs(str.charAt(0));
    if(!Number.isNaN(absVal)) {
        return absVal + this.sumDigitsR(str.substring(1));
    } else {
        return this.sumDigitsR(str.substring(1));
    }
}