Round Down Number

Question

How can I round down a number in Javascript?

math.round() doesn't work because it rounds it to the nearest decimal.

I'm not sure if there is a better way of doing it other than breaking it apart at the decimal point at keeping the first bit. There must be...

Answer 1

Math.floor()

is the answer.

Answer 2

So you want to truncate a double?

One way is to assign it to an int (or long).

double d = 12.3456;
int i = d; //i = 12

Answer 3

Math.floor(1+7/8)

Answer 4

Math.floor() will work, but it's very slow compared to using a bitwise OR operation:

var rounded = 34.923 | 0;
alert( rounded );
//alerts "34"

EDIT Math.floor() is not slower than using the | operator. Thanks to Jason S for checking my work.

Here's the code I used to test:

var a = [];
var time = new Date().getTime();
for( i = 0; i < 100000; i++ ) {
    //a.push( Math.random() * 100000  | 0 );
    a.push( Math.floor( Math.random() * 100000 ) );
}
var elapsed = new Date().getTime() - time;
alert( "elapsed time: " + elapsed );

Answer 5

Round towards negative infinity - Math.Floor()

+3.5 => +3.0
-3.5 => -4.0

Round towards zero - usually called Truncate(), but not supported by JavaScript. Can be emulated by using Math.Ceil() for negative numbers and Math.Floor() for positive numbers.

+3.5 => +3.0 using Math.Floor()
-3.5 => -3.0 using Math.Ceil()