How to use the d3-ease.easeLinear function in d3-ease

/**
 * Typescript definition tests for d3/d3-ease module
 *
 * Note: These tests are intended to test the definitions only
 * in the sense of typing and call signature consistency. They
 * are not intended as functional tests.
 */

import * as d3Ease from 'd3-ease';

let t_in: number = 0.5;
let t_out: number;

t_out = d3Ease.easeLinear(t_in);

t_out = d3Ease.easeQuad(t_in);
t_out = d3Ease.easeQuadIn(t_in);
t_out = d3Ease.easeQuadOut(t_in);
t_out = d3Ease.easeQuadInOut(t_in);

t_out = d3Ease.easeCubic(t_in);
t_out = d3Ease.easeCubicIn(t_in);
t_out = d3Ease.easeCubicOut(t_in);
t_out = d3Ease.easeCubicInOut(t_in);


let easePolyFactory: d3Ease.PolynomialEasingFactory;

easePolyFactory = d3Ease.easePoly;
easePolyFactory = d3Ease.easePoly.exponent(2);

return data.map(x => ({
    id: x.id,
    values: x.values.slice(0, Math.floor(x.values.length * easeLinear(t))),
  }));
};