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node-av / lib / Rational

Class: Rational ​

Defined in: src/lib/rational.ts:34

Rational number type for precise fractional representations.

Represents a rational number as a fraction (numerator/denominator). Used throughout FFmpeg for time bases, aspect ratios, frame rates, and other fractional values that require exact precision.

Example ​

typescript
import { Rational } from 'node-av';

// Common time bases
const timebase = new Rational(1, 90000); // 90kHz for MPEG-TS
const videoTimebase = new Rational(1, 25); // 25 fps
const audioTimebase = new Rational(1, 48000); // 48kHz audio

// Frame rates
const framerate = new Rational(30, 1);    // 30 fps
const ntscFramerate = new Rational(30000, 1001); // 29.97 fps NTSC
const palFramerate = new Rational(25, 1); // 25 fps PAL

// Aspect ratios
const aspectRatio = new Rational(16, 9); // 16:9
const pixelAspect = new Rational(1, 1); // Square pixels

// Arithmetic operations
const doubled = timebase.mul(new Rational(2, 1));
const inverted = framerate.inv(); // Get frame duration
const sum = framerate.add(new Rational(5, 1)); // Add 5 fps

Constructors ​

Constructor ​

new Rational(num, den): Rational

Defined in: src/lib/rational.ts:64

Create a new rational number.

Represents the fraction num/den.

Parameters ​

num ​

number

Numerator of the fraction

den ​

number

Denominator of the fraction (must not be 0)

Returns ​

Rational

Throws ​

If denominator is 0

Example ​

typescript
import { Rational } from 'node-av';

// Create time base for 25 fps video
const timebase = new Rational(1, 25);

// Create NTSC frame rate (29.97 fps)
const ntsc = new Rational(30000, 1001);

// Will throw error
try {
  const invalid = new Rational(1, 0);
} catch (error) {
  console.error('Cannot have zero denominator');
}

Properties ​

den ​

readonly den: number

Defined in: src/lib/rational.ts:66

Denominator of the fraction (must not be 0)

num ​

readonly num: number

Defined in: src/lib/rational.ts:65

Numerator of the fraction

Methods ​

add() ​

add(other): Rational

Defined in: src/lib/rational.ts:111

Add two rational numbers.

Performs addition: (a/b) + (c/d) = (ad + bc) / bd

Parameters ​

other ​

Rational

The rational number to add

Returns ​

Rational

A new Rational representing the sum

Example ​

typescript
const a = new Rational(1, 2); // 1/2
const b = new Rational(1, 3); // 1/3
const sum = a.add(b); // 5/6
console.log(sum.toString()); // "5/6"

div() ​

div(other): Rational

Defined in: src/lib/rational.ts:174

Divide two rational numbers.

Performs division: (a/b) ÷ (c/d) = (ad) / (bc)

Parameters ​

other ​

Rational

The rational number to divide by

Returns ​

Rational

A new Rational representing the quotient

Example ​

typescript
const pixels = new Rational(1920, 1); // 1920 pixels width
const aspect = new Rational(16, 9); // 16:9 aspect ratio
const height = pixels.div(aspect); // Calculate height
console.log(height.toDouble()); // 1080

equals() ​

equals(other): boolean

Defined in: src/lib/rational.ts:265

Check if this rational equals another.

Compares using cross-multiplication to avoid floating point errors. Two rationals a/b and c/d are equal if ad = bc.

Parameters ​

other ​

Rational

The rational to compare with

Returns ​

boolean

true if the rationals are equal, false otherwise

Example ​

typescript
const a = new Rational(2, 4);
const b = new Rational(1, 2);
const c = new Rational(3, 4);

console.log(a.equals(b)); // true (both are 1/2)
console.log(a.equals(c)); // false

inv() ​

inv(): Rational

Defined in: src/lib/rational.ts:217

Invert the rational number (reciprocal).

Returns the reciprocal: 1/(a/b) = b/a

Returns ​

Rational

A new Rational representing the reciprocal

Example ​

typescript
const framerate = new Rational(25, 1); // 25 fps
const frameDuration = framerate.inv(); // 1/25 seconds per frame
console.log(frameDuration.toString()); // "1/25"
console.log(frameDuration.toDouble()); // 0.04 seconds

isValid() ​

isValid(): boolean

Defined in: src/lib/rational.ts:198

Check if this rational is valid (both num and den are positive).

FFmpeg uses { 0, 0 } as "undefined" and { num, 0 } as infinity. This method returns true only if both values are positive.

Returns ​

boolean

true if num > 0 and den > 0

Example ​

typescript
const valid = new Rational(25, 1);
console.log(valid.isValid()); // true

const undefined = new Rational(0, 0);
console.log(undefined.isValid()); // false

const infinity = new Rational(1, 0);
console.log(infinity.isValid()); // false

mul() ​

mul(other): Rational

Defined in: src/lib/rational.ts:153

Multiply two rational numbers.

Performs multiplication: (a/b) × (c/d) = (ac) / (bd)

Parameters ​

other ​

Rational

The rational number to multiply by

Returns ​

Rational

A new Rational representing the product

Example ​

typescript
const framerate = new Rational(30, 1); // 30 fps
const duration = new Rational(5, 1); // 5 seconds
const frames = framerate.mul(duration); // 150/1 = 150 frames
console.log(frames.toDouble()); // 150

sub() ​

sub(other): Rational

Defined in: src/lib/rational.ts:132

Subtract two rational numbers.

Performs subtraction: (a/b) - (c/d) = (ad - bc) / bd

Parameters ​

other ​

Rational

The rational number to subtract

Returns ​

Rational

A new Rational representing the difference

Example ​

typescript
const a = new Rational(3, 4); // 3/4
const b = new Rational(1, 4); // 1/4
const diff = a.sub(b); // 2/4 = 1/2
console.log(diff.toString()); // "2/4"

toDouble() ​

toDouble(): number

Defined in: src/lib/rational.ts:241

Convert rational to floating point number.

Calculates the decimal value: num / den Note: This may lose precision for some rational values.

Returns ​

number

The floating point representation (Infinity if den = 0, NaN if both = 0)

Example ​

typescript
const ntsc = new Rational(30000, 1001); // NTSC frame rate
console.log(ntsc.toDouble()); // 29.97002997...

const half = new Rational(1, 2);
console.log(half.toDouble()); // 0.5

const infinity = new Rational(1, 0);
console.log(infinity.toDouble()); // Infinity

toString() ​

toString(): string

Defined in: src/lib/rational.ts:285

Get string representation of the rational.

Returns the rational in "num/den" format.

Returns ​

string

String representation as "numerator/denominator"

Example ​

typescript
const framerate = new Rational(30000, 1001);
console.log(framerate.toString()); // "30000/1001"

const timebase = new Rational(1, 90000);
console.log(`Timebase: ${timebase}`); // "Timebase: 1/90000"

fromObject() ​

static fromObject(rational): Rational

Defined in: src/lib/rational.ts:90

Create a Rational from an IRational object.

Parameters ​

rational ​

IRational

The IRational object with num and den properties

Returns ​

Rational

A new Rational instance

Example ​

typescript
import { Rational } from 'node-av';

const obj = { num: 3, den: 4 };
const rational = Rational.fromObject(obj);
console.log(rational.toString()); // "3/4"