inverseLerp

Calculates where a number value lies between two numbers start and stop, as a fraction from 0 to 1. This is the opposite of lerp.

const fraction = inverseLerp(start, stop, value);

Usage

inverseLerp(start, stop, value)

Use inverseLerp when you treat start to stop as one range and want to know how far along it value is. When value is start you get 0, when it is stop you get 1, and when it is halfway between them you get 0.5. This is useful for turning a scroll position or slider value into a progress value from 0 to 1.

import { inverseLerp } from 'es-toolkit/math';

// 50 is halfway between 0 and 100
inverseLerp(0, 100, 50);
// Returns: 0.5

// 12.5 is a quarter of the way from 10 to 20
inverseLerp(10, 20, 12.5);
// Returns: 0.25

// A value of start gives 0, a value of stop gives 1
inverseLerp(0, 100, 0);
// Returns: 0
inverseLerp(0, 100, 100);
// Returns: 1

// start can be greater than stop
inverseLerp(100, 0, 75);
// Returns: 0.25

When value falls outside start and stop, the result is also less than 0 or greater than 1. If you only want a fraction between 0 and 1, apply clamp to the result.

import { clamp, inverseLerp } from 'es-toolkit/math';

// A value past stop gives a fraction greater than 1
inverseLerp(0, 100, 150);
// Returns: 1.5

// Keep the result between 0 and 1
clamp(inverseLerp(0, 100, 150), 0, 1);
// Returns: 1

When start and stop are the same number there is no range to measure against, so 0 is returned.

import { inverseLerp } from 'es-toolkit/math';

// The range has zero length
inverseLerp(5, 5, 5);
// Returns: 0

Combined with lerp, you can move a number from one range to another.

import { inverseLerp, lerp } from 'es-toolkit/math';

// Move 150 from [100, 200] to [0, 1000]
lerp(0, 1000, inverseLerp(100, 200, 150));
// Returns: 500

Parameters

• start (number): The start of the range. Returns 0 when value equals this.
• stop (number): The end of the range. Returns 1 when value equals this.
• value (number): The number to locate within the range.

Returns

(number): Where value lies between start and stop, as a fraction from 0 to 1.