Why Does `Array.map()` with `parseInt()` Produce Unexpected Results in JavaScript?

Parsing Numbers in Array.map() with parseInt

In Javascript, the Array.map() function applies a callback function to each element of an array. However, the parseInt() function, when used in this context, can yield unexpected results.

In the provided example:

['1','2','3'].map(parseInt);

parseInt() yields [1, NaN, NaN] instead of the expected [1, 2, 3]. This behavior arises because parseInt() takes two arguments: the string to parse and an optional radix (base).

The Array.map() callback function provides three parameters: the element value, its index, and the array itself. When using parseInt() without specifying a radix, it uses the second parameter (the index) as the radix.

For ['1','2','3'], the radix values used by parseInt() become 0, 1, and 2. Since radix 1 is invalid and 3 is not representable in binary (radix 2), the parsing fails, resulting in NaN.

To correct this behavior, provide an explicit radix to parseInt() within a wrapper function:

['1','2','3'].map(function(num) { return parseInt(num, 10); });

Alternatively, use the ES2015 syntax:

['1','2','3'].map(num => parseInt(num, 10));

By specifying a radix of 10, we ensure that parseInt() correctly parses the strings into integers.

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