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Confusions about parseInt

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Backgrounds

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parseInt(0.0000005)
// 5

Explanations

​ parseInt accepts string as the first argument, represents the value to parse. If this argument is not a string, then it is converted to one using the ToString abstract operation.

​ In JavaScript, the number which has more than 6 zeros in front of the first non-zero digit will be displayed in scientific notation, specified by ECMA . That means , 0.00000005.toString() will return '5e-7'

​ So, the code above can be explained as following:

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0.0000005.toString()
// '5e-7'

parseInt(5e-7)
// 5
parseInt('5e-7')
// 5

Extra

What about postive numbers ?

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parseInt(5e7)
parseInt(50000000)
// both return 50000000

// 5e7 will not be displayed in scientific notation
5e7.toString()
// '50000000'

// '5e6' is string
parseInt('5e6')
// 5

Tips

parseInt should not be used as a substitute for Math.floor()