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JavaScript - Math.asin() method example

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Root-ssh

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The Math.asin function returns number in radians in the range -Math.PI/2 to +Math.PI/2. The function calculates inverted sine function value.

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function calculateAngle(a, h) {
    return Math.asin(a / h);
}
​
/*
  |\
  | \ h
a |  \
  |__*\ <- angle
    b
*/
​
var a, b, h;
​
// a, b and h build isosceles right triangle
a = 3; 
b = a;
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(a, h) ); // 0.7853981633974483 <- ~45 degrees
​
// a, b and h build half of equilateral triangle
a = 3; 
b = a * Math.sqrt(3); 
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(a, h) ); // 0.5235987755982989 <- ~30 degrees
​
// a, b and h are not able to build triangle
a = 3; 
b = a; 
h = 1;
console.log( calculateAngle(a, h) ); // NaN

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1. Documentation

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Syntax	`Math.asin(number)`
Parameters	`number` - integer or float number value that represents result of operation `adjacent / hypotenuse` on right triangle (primitive value). `number` should be in the range `-1` to `+1`.
Result	`number` value in radians in the range `-Math.PI/2` to `+Math.PI/2` (primitive value). If value can not be calculated `NaN` is returned.
Description	`asin` is a static method that takes only one parameter and returns an approximation of the result of the mathematical function arcsine(x).

2. Working with degrees

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function calculateAngle(a, h) {
    var angle = Math.asin(a / h);
​
    return (180 / Math.PI) * angle; // rad to deg conversion
}
​
/*
  |\
  | \ h
a |  \
  |__*\ <- angle
    b
*/
​
var a, b, h;
​
// a, b and h build isosceles right triangle
a = 3; 
b = a;
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(a, h) ); // ~45 degrees
​
// a, b and h build half of equilateral triangle
a = 3; 
b = a * Math.sqrt(3); 
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(a, h) ); // ~30 degrees
​
// a, b and h are not able to build triangle
a = 3; 
b = a; 
h = 1;
console.log( calculateAngle(a, h) ); // NaN

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3. Canvas plot example

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<!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');
​
    // arcsine chart range
    var x1 = -1.0;
    var x2 = +1.0;
    var y1 = -Math.PI / 2;
    var y2 = +Math.PI / 2;
​
    var dx = 0.01;
​
    var xRange = x2 - x1;
    var yRange = y2 - y1;
​
    function calculatePoint(x) {
      var y = Math.asin(x);
​
      // chart will be reversed horizontaly 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 + '> // angles in radians');
​
    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>