为什么在JavaScript中将“ 1.0”传递给函数将其更改为“ 1”

Question

Can someone explain to me why when pass 1.0 to a function in javascript it gets converted to 1 and how to work around this quirk?

var return_me = function(value) {
    return value;
}

console.log("1.0 is returned as " + return_me(1.0));

Answer 1

JavaScript doesn't distinguish between int or float like other more strongly typed languages. It just has one Number type. From the ECMA specifications :

Once the exact mathematical value (MV) for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 8.5), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

• it is not 0;

• or there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

More info on Number .

So basically, the answer is that JavaScript will display numbers that look like integers as integers and numbers that look like floats as floats.

Answer 2

In javascript there are six build in types of values.

1. string
2. number
3. boolean
4. null and undefined
5. object
6. symbol

These are mentioned to the book You don't know JS which I find really useful in my effort to learn javascript.

As a result js sees the var value of your function as a typeof number and understands that 1.0 is the same as 1. (in case the 1.0 was 1.9 it returns 1.9 as expected).

Now if you want to keep these decimals (even if there are zero digits) you could pass the value as a string.

console.log("1.0 is returned as " + return_me("1.0"));