How can I round down a number in Javascript?

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...

Using Math.floor() is one way of doing this.

More information: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/floor

Round towards negative infinity - Math.floor()

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

Round towards zero can be done using Math.trunc(). Older browsers do not support this function. If you need to support these, you can use 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()

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 );

You can try to use this function if you need to round down to a specific number of decimal places

function roundDown(number, decimals) {
    decimals = decimals || 0;
    return ( Math.floor( number * Math.pow(10, decimals) ) / Math.pow(10, decimals) );
}

examples

alert(roundDown(999.999999)); // 999
alert(roundDown(999.999999, 3)); // 999.999
alert(roundDown(999.999999, -1)); // 990

Rounding a number towards 0 (aka "truncating its fractional part") can be done by subtracting its signed fractional part number % 1:

rounded = number - number % 1;

Like Math.floor (rounds towards -Infinity) this method is perfectly accurate.

There are differences in the handling of -0, +Infinity and -Infinity though:

Math.floor(-0) => -0
-0 - -0 % 1    => +0

Math.floor(Infinity)    => Infinity
Infinity - Infinity % 1 => NaN

Math.floor(-Infinity)     => -Infinity
-Infinity - -Infinity % 1 => NaN

Math.floor(1+7/8)

To round down towards negative infinity, use:

rounded=Math.floor(number);

To round down towards zero (if the number can round to a 32-bit integer between -2147483648 and 2147483647), use:

rounded=number|0;

To round down towards zero (for any number), use:

if(number>0)rounded=Math.floor(number);else rounded=Math.ceil(number);

Was fiddling round with someone elses code today and found the following which seems rounds down as well:

var dec = 12.3453465,
int = dec >> 0; // returns 12

For more info on the Sign-propagating right shift(>>) see MDN Bitwise Operators

It took me a while to work out what this was doing :D

But as highlighted above, Math.floor() works and looks more readable in my opinion.

This was the best solution I found that works reliably.

function round(value, decimals) {
    return Number(Math.floor(parseFloat(value + 'e' + decimals)) + 'e-' + decimals);
 }

Credit to: Jack L Moore's blog

You need to put -1 to round half down and after that multiply by -1 like the example down bellow.

<script type="text/javascript">

  function roundNumber(number, precision, isDown) {
    var factor = Math.pow(10, precision);
    var tempNumber = number * factor;
    var roundedTempNumber = 0;
    if (isDown) {
      tempNumber = -tempNumber;
      roundedTempNumber = Math.round(tempNumber) * -1;
    } else {
      roundedTempNumber = Math.round(tempNumber);
    }
    return roundedTempNumber / factor;
  }
</script>

<div class="col-sm-12">
  <p>Round number 1.25 down: <script>document.write(roundNumber(1.25, 1, true));</script>
  </p>
  <p>Round number 1.25 up: <script>document.write(roundNumber(1.25, 1, false));</script></p>
</div>

Here is math.floor being used in a simple example. This might help a new developer to get an idea how to use it in a function and what it does. Hope it helps!

<script>

var marks = 0;

function getRandomNumbers(){	//	generate a random number between 1 & 10
	var number = Math.floor((Math.random() * 10) + 1);
	return number;
}

function getNew(){	
/*	
	This function can create a new problem by generating two random numbers. When the page is loading as the first time, this function is executed with the onload event and the onclick event of "new" button.
*/
document.getElementById("ans").focus();
var num1 = getRandomNumbers();
var num2 = getRandomNumbers();
document.getElementById("num1").value = num1;
document.getElementById("num2").value = num2;
	
document.getElementById("ans").value ="";
document.getElementById("resultBox").style.backgroundColor = "maroon"
document.getElementById("resultBox").innerHTML = "***"
	
}

function checkAns(){
/*
	After entering the answer, the entered answer will be compared with the correct answer. 
		If the answer is correct, the text of the result box should be "Correct" with a green background and 10 marks should be added to the total marks.
		If the answer is incorrect, the text of the result box should be "Incorrect" with a red background and 3 marks should be deducted from the total.
		The updated total marks should be always displayed at the total marks box.
*/
	
var num1 = eval(document.getElementById("num1").value);
var num2 = eval(document.getElementById("num2").value);
var answer = eval(document.getElementById("ans").value);

if(answer==(num1+num2)){
	marks = marks + 10;
	document.getElementById("resultBox").innerHTML = "Correct";
	document.getElementById("resultBox").style.backgroundColor = "green";
	document.getElementById("totalMarks").innerHTML= "Total marks : " + marks;

}

else{
	marks = marks - 3;
	document.getElementById("resultBox").innerHTML = "Wrong";
	document.getElementById("resultBox").style.backgroundColor = "red";
	document.getElementById("totalMarks").innerHTML = "Total Marks: " + marks ;
}

	
	
	
}

</script>
</head>

<body onLoad="getNew()">
	<div class="container">
    	<h1>Let's add numbers</h1>
        <div class="sum">
	        <input id="num1" type="text" readonly> + <input id="num2" type="text" readonly>
        </div>
        <h2>Enter the answer below and click 'Check'</h2>
        <div class="answer">
	        <input id="ans" type="text" value="">
        </div>
        <input id="btnchk" onClick="checkAns()" type="button" value="Check" >
        <div id="resultBox">***</div>
        <input id="btnnew" onClick="getNew()" type="button" value="New">
        <div id="totalMarks">Total marks : 0</div>  
    </div>
</body>
</html>

Math.round(3.14159 * 100) / 100  // 3.14

3.14159.toFixed(2);              // 3.14 returns a string
parseFloat(3.14159.toFixed(2));  // 3.14 returns a number

Math.round(3.14159)  // 3
Math.round(3.5)      // 4
Math.floor(3.8)      // 3
Math.ceil(3.2)       // 4