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JavaScript - Math.round() method example

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Created by:
Eshaal-Wilkinson
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Math round is a static method that returns a number that is rounded to the closest integer number.

// ONLINE-RUNNER:browser;

console.log( Math.round(  5     ) ); //  5

console.log( Math.round(  2.49  ) ); //  2
console.log( Math.round(  2.50  ) ); //  3
console.log( Math.round(  2.51  ) ); //  3

console.log( Math.round( -2.49  ) ); // -2
console.log( Math.round( -2.50  ) ); // -2
console.log( Math.round( -2.51  ) ); // -3

console.log( Math.round(  0.999 ) ); //  1
console.log( Math.round(  1.001 ) ); //  1
console.log( Math.round( -1.001 ) ); // -1

1. Documentation

SyntaxMath.round(number)
Parametersnumber - integer or float number value (primitive value).
ResultRounded number value (primitive value).
Descriptionround is a static method that returns a number that is rounded to the closest integer number.

2. Custom round method examples

2.1. Rounding with precision to n places example

This section contains a custom function that shows how to round numbers with precision to n places.

// ONLINE-RUNNER:browser;

function roundPrecised(number, precision) {
	var power = Math.pow(10, precision);
  	return Math.round(number * power) / power;
}


// Usage example:

console.log( roundPrecised(     5  ,  0 ) ); // 5
console.log( roundPrecised(     5. ,  0 ) ); // 5
console.log( roundPrecised(      .5,  0 ) ); // 1

console.log( roundPrecised(  1.2345,  0 ) ); // 1
console.log( roundPrecised(  1.2345,  1 ) ); // 1.2
console.log( roundPrecised(  1.2345,  2 ) ); // 1.23
console.log( roundPrecised(  1.2345,  3 ) ); // 1.235

console.log( roundPrecised( -1.2345,  0 ) ); // -1
console.log( roundPrecised( -1.2345,  1 ) ); // -1.2
console.log( roundPrecised( -1.2345,  2 ) ); // -1.23
console.log( roundPrecised( -1.2345,  3 ) ); // -1.234

console.log( roundPrecised(   12345, -1 ) ); // 12350
console.log( roundPrecised(   12345, -2 ) ); // 12300
console.log( roundPrecised(   12345, -3 ) ); // 12000

2.2. VanillaJS round implementation example

This section contains custom round function implementation.

// ONLINE-RUNNER:browser;

function roundNumber(value) {
  	if (value < 0.0) {
      	var rest = (value % 1.0);
      	if(rest < -0.5) {
        	rest += 1.0;
        }
    	return value - rest;
    } else {
    	value += 0.5;
    	return value - (value % 1.0);
  	}
}


// Usage example:

console.log( roundNumber(  5     ) ); //  5

console.log( roundNumber(  2.49  ) ); //  2
console.log( roundNumber(  2.50  ) ); //  3
console.log( roundNumber(  2.51  ) ); //  3

console.log( roundNumber(  0.999 ) ); //  1
console.log( roundNumber(  1.001 ) ); //  1

console.log( roundNumber( -2.49  ) ); // -2
console.log( roundNumber( -2.50  ) ); // -2
console.log( roundNumber( -2.51  ) ); // -3

console.log( roundNumber( -1.001 ) ); // -1

3. Custom plot example

A simple plot with canvas is presented for Math.round function below.

// ONLINE-RUNNER:browser;

<!doctype html>
<html>
<head>
  <style> #canvas { border: 1px solid black; } </style>
</head>
<body>
  <canvas id="canvas" width="200" height="200"></canvas>
  <script>
    
    var canvas = document.querySelector('#canvas');
    var context = canvas.getContext('2d');

    // cosine chart range
    var x1 = -10.0;
    var x2 = +10.0;
    var y1 = -10.0;
    var y2 = +10.0;

    var dx = 0.01;

    var xRange = x2 - x1;
    var yRange = y2 - y1;

    function calculatePoint(x) {
        var y = Math.round(x);
	  
        // chart will be reversed horizontally because of reversed canvas pixels
	  
        var nx = (x - x1) / xRange;       // normalized x
        var ny = 1.0 - (y - y1) / yRange; // normalized y
        
        var point = {
            x: nx * canvas.width,
            y: ny * canvas.height
        };
	  
        return point;
    }

    console.log('x range: <' + x1 + '; ' + x2 + '>');
    console.log('y range: <' + y1 + '; ' + y2 + '>');

    var point = calculatePoint(x1);
    
    context.beginPath();
    context.moveTo(point.x, point.y);

    for (var x = x1 + dx; x < x2; x += dx) {
        point = calculatePoint(x);
        context.lineTo(point.x, point.y);
    }

    point = calculatePoint(x2);
    context.lineTo(point.x, point.y);
    context.stroke();

  </script>
</body>
</html>

Alternative titles

  1. JavaScript - Math.round() documentation with examples
  2. js - Math.round() method documentation with examples
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JavaScript - Math documentation (EN)

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