force.js

let mags = [{
        mag: 0,
        angle: 0
    },
    {
        mag: 0,
        angle: 0
    },
    {
        mag: 0,
        angle: 0
    },
    {
        mag: 0,
        angle: 0
    }
]

function calcResultant() {
    let resultant = {
        mag: null,
        angle: null,
        x: 0,
        y: 0,
        color: '#f00'
    }
    mags.forEach(el => {
        resultant.x += calcX(el.angle, el.mag)
        resultant.y += calcY(el.angle, el.mag)
    }, this)
    resultant.mag = pythag(resultant.x, resultant.y)
    resultant.angle = atand(resultant.y / resultant.x)
    resultant.angle = resultant.angle < 0 ? 360 + resultant.angle : resultant.angle
    resultant.angle += 180
    return resultant
}

function calcX(angle, magnitude) {
    return cosd(angle) * magnitude
}

function calcY(angle, magnitude) {
    return sind(angle) * magnitude
}

function sind(deg) {
    return Math.sin(deg * (Math.PI / 180))
}

function cosd(deg) {
    return Math.cos(deg * (Math.PI / 180))
}

function tand(deg) {
    return Math.tan(deg * (Math.PI / 180))
}

function atand(deg) {
    return Math.atan(deg) * (180/ Math.PI)
}

function pythag(a, b) {
    return Math.sqrt(Math.pow(a, 2) + Math.pow(b, 2))
}

draw(mags)