I'm new to game development and am looking for some help on improving my jump handling for a simple side scrolling game I've made. I would like to make the jump last longer if the key is held down for the full length of the jump, otherwise if the key is tapped, make the jump not as long. Currently, how I'm handling the jumping is the following:

Player.prototype.jump = function () {
    // Player pressed jump key
    if (this.isJumping === true)
    {
        // Set sprite to jump state
        this.settings.slice = 250;

        if (this.isFalling === true)
        {
            // Player let go of jump key, increase rate of fall
            this.settings.y -= this.velocity;
            this.velocity -= this.settings.gravity * 2;
        }
        else
        {
            // Player is holding down jump key
            this.settings.y -= this.velocity;
            this.velocity -= this.settings.gravity;
        }

    }

    if (this.settings.y >= 240)
    {
        // Player is on the ground
        this.isJumping = false;
        this.isFalling = false;
        this.velocity = this.settings.maxVelocity;
        this.settings.y = 240;
    }
}

I'm setting isJumping on keydown and isFalling on keyup. While it works okay for simple use, I'm looking for a better way handle jumping and gravity. It's a bit buggy if the gravity is increased (which is why I had to put the last y setting in the last if condition in there) on keyup, so I'd like to know a better way to do it. Where are some resources I could look at that would help me better understand how to handle jumping and gravity? What's a better approach to handling this? Like I said, I'm new to game development so I could be doing it completely wrong. Any help would be appreciated.