goalseek
goalseek is a javascript library that can be used to solve for the value of an independent variable: "x"; of a function: "f(x)"; such that f(x) equals some defined goal. In other words: do you know the desired output of a function but not the input to yield such an output? If so, then use this goal seek!
Currently, this goal seek uses Steffensen's Method to find the root of the error.
Installation
goalseek
is an npm package, so installation is as easy as:
$ npm install save goalseek
Usage
The package exports two error types, the function parameter type and one function, goalSeek
as a default export:
export const IsNanError = TypeError('resulted in NaN');
export const FailedToConvergeError = Error('failed to converge');
export const InvalidInputsError = Error('invalid inputs');
export type Params = {
fn: (...inputs: any[]) => number;
fnParams: any[];
percentTolerance?: number;
customToleranceFn?: (arg0: number) => boolean;
maxIterations: number;
maxStep: number;
goal: number;
independentVariableIdx: number;
};
const goalSeek = ({
fn,
fnParams,
percentTolerance,
maxIterations,
maxStep,
goal,
independentVariableIdx,
}: Params): number => {
...
}
export default goalSeek;
The goalSeek
function takes one object argument with the following keys:

fn
 the function, "f(x)" that is being evaluated. 
fnParams
 an array of parameters that are to be used as inputs tofn
. 
percentTolerance
 the acceptable error range to the stated goal. For example, ifgoal: 100
andpercentTolerance: 1
, then any values in the range [99, 101] will be accepted as correct (± 1% of 100). If used,customToleranceFn
should be left undefined. 
customToleranceFn
 a custom function that can be used to check the validity of a result. If used,percentTolerance
should be left undefined. 
maxIterations
 the maximum number of attempts to make. 
maxStep
 the maximum magnitude step size to move the independent variablex
for the next guess. 
goal
 the desired output of thefn
. 
independentVariableIdx
 the index position of the independent variablex
in thefnParams
array.
To use the function, for example, with a simple linear equeation:
let x = 10;
const fn = (x: number): number => x + 2;
const fnParams = [x];
try {
const result = goalSeek({
fn,
fnParams,
percentTolerance: 1,
maxIterations: 1000,
maxStep: 10,
goal: 100,
independentVariableIdx: 0
})
// result => 98
} catch(e) {
}
Errors
This library will throw one of two errors: IsNanError
or FailedToConvergeError
.
IsNanError
IsNanError
is thrown whenever the result of fn
returns a value that is not a number. For example, if fn
is Math.log
, and the value 1
is input goalSeek
with throw IsNanError
.
FailedToConvergeError
FailedToConvergeError
is thrown when no acceptable independent variable can be found. For example, if fn
is x * x
and goal is 1
the library will fail to converge and throw FailedToConvergeError
.
Examples
import goalSeek from 'goalseek';
const fn = (x,y,z) => x * y * z;
const fnParams = [4,5,6];
try {
const result = goalSeek({
fn,
fnParams,
percentTolerance: 1,
maxIterations: 1000,
maxStep: 1,
goal: 140,
independentVariableIdx: 2
});
console.log(`result: ${result}`);
} catch (e) {
console.log('error', e);
}
// result: 7
import goalSeek from 'goalseek';
const fn = (x,y,z) => x * y * z;
const fnParams = [4,5,6];
const customToleranceFn = (x: number): boolean => {
return x < 1
}
try {
const result = goalSeek({
fn,
fnParams,
customToleranceFn,
maxIterations: 1000,
maxStep: 0.01,
goal: 0.5,
independentVariableIdx: 2
});
console.log(`result: ${result}`);
} catch (e) {
console.log('error', e);
}
// result: 7
Licenses:
This work is licensed under MIT.
The MIT License (MIT)
Copyright (c) 2020 Adam Hanna
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.