Follow the movement of an object

vector.js

/**
 * Create an immutable vector object. There's no assumption made on what
 * the "unit" and hence the meaning of the vector is.
 *
 * @param (Number) x - vector value in x-direction
 * @param (Number) y - vector value in y-direction
 * @return (Object)
 */
function vector(x, y)
{
    var obj = Object.create(vector.prototype)
    Object.defineProperty(obj, 'x', { value: +x.toFixed(vector.scale) })
    Object.defineProperty(obj, 'y', { value: +y.toFixed(vector.scale) })
    return obj
}
// account for floating-point math inaccuracy
vector.scale = 6
// define how x/y change over time
vector.prototype.delta = false
// calculate the next trajectory point
vector.prototype.next = function (time=0) {
    if (!this.delta) {
        return this
    }
    var delta = this.delta.next(time)
    var obj = this.add(delta.mult(time))
    obj.delta = delta
    return obj
}
// vector math - addition
vector.prototype.add = function (vec) {
    return vector(this.x + vec.x, this.y + vec.y)
}
// vector math - multiplication
vector.prototype.mult = function (factor) {
    return vector(this.x * factor, this.y * factor)
}
// vector math - vector length
vector.prototype.valueOf = function () {
    return +Math.sqrt(this.x * this.x + this.y * this.y).toFixed(vector.scale)
}
// textual representation
vector.prototype.toString = function () {
    return `(${this.x}, ${this.y})`
}

/**
 * Example: the trajectory of a moving object under the influence of gravity
 */

// object position
var obj = vector(10, 10)
// object speed
var speed = vector(10, 5)
// object acceleration
var gravity = vector(0, -9.81)
// the object's position is changed by its current speed
obj.delta = speed
// the object's speed is altered by the gravity
speed.delta = gravity

while (obj.y >= 0) {
    // work well only if the interval is sufficiently small => cf. differential calculus
    obj = obj.next(0.001)
    console.log(obj.x, obj.y)
}