Question

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javascript – Array.map and parseInt

I stumbled across the following code snippet:

> ['10','10','10','10','10'].map(parseInt);
[10, NaN, 2, 3, 4]

What’s happening here?

Answer 1

Start by consulting the documentation for Array.prototype.map. The key is this line:

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

(emphasis mine)

Then check the documentation for parseInt:

The parseInt function converts its first argument to a string, parses it, and returns an integer or NaN. If not NaN, the returned value will be the decimal integer representation of the first argument taken as a number in the specified radix (base). For example, a radix of 10 indicates to convert from a decimal number, 8 octal, 16 hexadecimal, and so on. For radices above 10, the letters of the alphabet indicate numerals greater than 9. For example, for hexadecimal numbers (base 16), A through F are used.

and:

If radix is undefined or 0, JavaScript assumes the following:

• If the input string begins with "0x" or "0X", radix is 16 (hexadecimal).

• If the input string begins with "0", radix is eight (octal). This feature is non-standard, and some implementations deliberately do not support it (instead using the radix 10). For this reason always specify a radix when using parseInt.

• If the input string begins with any other value, the radix is 10 (decimal).

So the first call is:

parseInt('10',0, ['10','10',...]) // => 10 (because radix=0)

the second is:

parseInt('10',1, ['10','10',...]) // => NaN because radix is 1

the third:

parseInt('10',2, ['10','10',...]) // => 2 because 10 in binary is the number "2"

and so on.