Javascript.md

Table of Contents

• Javascript
  • Language gotchas
    • Javascript breaks transitivity
    • Parsing integers

Javascript

• Many other examples here: wtfjs

Language gotchas

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Javascript breaks transitivity

document.write("'0'",'0'==0?"==":"<>","0 and ");
document.write("0",0=='0.0'?"==":"<>","'0.0' and ");
document.write("'0'",'0'=='0.0'?"==":"<>","'0.0'<br />");

Expected result: '0'==0 and 0=='0.0' and '0'=='0.0'

Result: '0'==0 and 0=='0.0' and '0'<>'0.0'

Reason: Strings can be casted to integers, except if compared to another string

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Parsing integers

The parseInt function seems to quite innovative:

parseInt('2',10)
2
parseInt('null',10)
NaN
parseInt('null',23)
NaN
parseInt('null',24)
23
parseInt('infinity',20)
18
parseInt('infinity',28)
324267766

Expected result: NaN

Reason: ???

Theory: In base 24, 0-9 are the first 10 digits. Then come the letters. And since "n" is the 14th letter, it has a value in base 24 Since "n" can be interpreted, it moves on to the next letter: "u". But "u" doesn't have a value in base 24, since there are only 24 digits! So, the parseInt function just returns the integer value for what "n" represents in base 24 → 23.

This would work for the 'null' string, but has no explanation for the null object:

parseInt(null,24)
23

Also, this seems to stop working at base 37:

parseInt("null",36)
1112745
parseInt("null",37)
NaN

See https://tc39.github.io/ecma262/#sec-parseint-string-radix for the definition of parseint.

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