问题
console.log(2E-12); // returns 2E-12
console.log(2E12); // returns 2000000000000
Why does line one return 2E-12 and not the same as line two. Is this an illegal way of using the exponent?
回答1:
From the ECMAScript specification of toString applied to the Number type:
7. If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal representation of s, followed by a decimal point ‘.’, followed by the remaining k−n digits of the decimal representation of s.
8. If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point ‘.’, followed by −n occurrences of the character ‘0’, followed by the k digits of the decimal representation of s.
9. Otherwise, if k = 1, return the String consisting of the single digit of s, followed by lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).
n is effectively the exponent of the number. So this says that if the exponent is between -7 and 21 the number should be displayed normally, otherwise exponential notation should be used.
回答2:
The renderer is being clever. If you try console.log(2E-6), you'll see it does what you're expecting. console.log(2E-7) does not...at least in Chrome and the current version of IE. Someone decided exponential notation was more legible for long fractional values.
Additionally, note that the 2e-12 you're getting is a number, not a string.
> console.log(2e-12*2e5)
4e-7
来源：https://stackoverflow.com/questions/25374051/javascript-number-literal-using-exponent