unexpected output in javascript when converting string to number

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.

It fails after 20 digit integer number with radix 10.

You may have a look to the description of parseInt:

Because some numbers include the e character in their string representation (e.g. 6.022e23), using parseInt to truncate numeric values will produce unexpected results when used on very large or very small numbers. parseInt should not be used as a substitute for Math.floor().

From the standard ECMA 252 V 5.1 15.1.2.2 parseInt (string , radix)

Step 13:

Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)

...

NOTE

parseInt may interpret only a leading portion of string as an integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no indication is given that any such characters were ignored.

var x = 5.7 * 1e20;
console.log(x);
console.log(parseInt(x, 10));

x = 5.7 * 1e21;
console.log(x);
console.log(parseInt(x, 10));