JS Gotchas! (map with parseInt)

Dimritium — Fri, 18 Dec 2020 16:46:48 +0000

In this series we would be discussing on some of the gotchas that I encounter while working with JavaScript. As evident from the title, in this one we would be looking at a problem statement related to Array.prototype.map and parseInt.

Note: Please do not confuse with Map object that holds key value pair, I will refer Array.prototype.mapas map in this article

The problem statement:

Let’s try to figure out the output of the following snippet:

let date = “2020–12–12”;  
let res = date.split(“-”).map(parseInt);  
console.log(res);

The output:

[2020, NaN, 1]

Interesting, isn’t it? 2020 was as expected but what happened to other elements of res ? Why the other elements failed to parse as integers?

Now lets dive into how JS interpreted the statement.

When we apply map operation on an array in JS, it expects a callback function and that callback function we have provided that is parseInt. In addition to that map always passes 3 arguments to the callback function. These 3 arguments being:

The element itself
index of the element
the whole array

If you want to confirm this you can run the following snippet in the console:

[2, 3, 4].map((x, y, z) => console.log(x, y, z))

The output will be:

2 0 [2, 3, 4]  
3 1 [2, 3, 4]  
4 2 [2, 3, 4]

Thus it is clear that this is the way map works.

Now let’s get back to our initial problem statement

The parseInt accepts 2 agruments:

string: the value to parse
radix(optional): an integer between 2 and 36 denoting the base in mathematical numeral system

Since map passes the 3 arguments, the first (the element) and second (element’s index) is picked up by the parseInt but the third (the array) is neglected.

Thus the radix value is being set as the index of the element. Thus in our initial problem, following iterations are performed for parseInt:

//first iteration  
parseInt("2020", 0);

//second iteration  
parseInt("12", 1);      

//third iteration:  
parseInt("12", 2);

And since we need to have radix as 10 (if working with decimal number system), our expected result is not met.

The solution (maybe):

// modified res  
res = date.split(“-”).map((elem) => parseInt(elem));

Note: Be careful with the above solution as parseInt implementation is up to the browser and not all browser have default radix set to 10, thus it is always advised to pass radix value whenever using parseInt

DEV Community: Dimritium

JS Gotchas! (map with parseInt)

The problem statement:

The output:

The solution (maybe):

Other Solutions: