unexpected output in javascript when converting string to number

Question

In MDN in this example on parseInt method

console.log(parseInt(4.7 * 1e22, 10)); // Very large number becomes 4
console.log(parseInt(4.7 * 1e20, 10)); //result is 470000000000000000000

or small number than 20 it give me expected result what is reason for this ?

Answer 1

With help from @Xufox

console.log(parseInt(4.7 * 1e22, 10)); // Very large number becomes 4
console.log(parseInt(4.7 * 1e20, 10)); //result is 470000000000000000000

What's happening here in steps:

• Calculation is done (4.7 * 1e20) and (4.7 * 1e22)
• Result of calculation is stringified by the JavaScript engine so it can be passed to parseInt
• The string is parsed back to a number
• Finally it's logged

JavaScript truncates every number with more than 20 digits using the scientific notation. That means the result of the calculations are:

• 470000000000000000000
• 4.7e22

These are stringified before being passed to parseInt:

• "470000000000000000000"
• "4.7e22"

These are strings, not numbers. parseInt will now ignore everything after the dot in the second value and return 4.