d3.interpolateNumber()

d3.interpolateNumber(a, b)

Returns an interpolator between the two numbers a and b. The returned interpolator is equivalent to:

function interpolator(t) {
  return a * (1 - t) + b * t;
}

Caution: avoid interpolating to or from the number zero when the interpolator is used to generate a string. When very small values are stringified, they may be converted to scientific notation, which is an invalid attribute or style property value in older browsers. For example, the number 0.0000001 is converted to the string "1e-7". This is particularly noticeable with interpolating opacity. To avoid scientific notation, start or end the transition at 1e-6: the smallest value that is not stringified in scientific notation.