.map(parseInt) does not do what it says on the tin

The Context

Writing a script for a pre-push hook which uses a PowerShell script to check for occurrences of certain strings (e.g. @ts-ignore and @ts-expect-error) being removed from the codebase. We can then gamify tech quality! ✨

I’m running the PowerShell script from JavaScript with execSync. The PowerShell script returns a comma separated list of string numbers (e.g. ['1']). In processing this list, I ran into an issue that I had experienced a while ago, but forgot about when trying to convert to a list of numbers using .map(parseInt).

The Problem


const strArr = ['2', '2', '1', '1'];
const intArr = strArr.map(parseInt);

console.log(intArr); // >> [2, NaN, 1, 1]

I initially expected the output of this (from reading the syntax) to be [2, 2, 1, 1].

The Solution

The API for parseInt, specifies that the parameters it takes are:

string
A string starting with an integer. Leading whitespace in this argument is ignored.
radix Optional
An integer between 2 and 36 that represents the radix (the base in mathematical numeral systems) of the string. It is converted to a 32-bit integer; if it's nonzero and outside the range of [2, 36] after conversion, the function will always return NaN. If 0 or not provided, the radix will be inferred based on string's value. Be careful — this does not always default to 10! The description below explains in more detail what happens when radix is not provided.

The API for .map exposes the following parameters:

element
The current element being processed in the array.
index
The index of the current element being processed in the array.
array
The array map() was called upon.

This means that my original code is the equivalent of:


const intArr = strArr.map((...args) => parseInt(...args));
const intArr = strArr.map((element, index, array) => parseInt(element, index, array));

Which passes through the index from map as the optional radix parameter in parseInt.

The Fix (easy)


// Fixed
const intArr = strArr.map((element) => parseInt(element));