vec3.lerp

vec3.lerp takes a, b and t.

vec3.lerp(a, b, t)

function update(dt) {  vec3.lerp(1, 0.4, 0.5);}

vec3.lerp is called with values of the shape its signature asks for. The code sits in update(dt), which the engine calls every frame: dt is the time since the last one, in seconds.