How do i code jumping for a 2d game in JS?

Here's a start:

 var maxwidth : float = 1;     
 var movespeed : float = 0;
 var jump : int = 4;
 var canjump = true;
 var maxheight : float = 6;
 var anim;
 
 function Start () 
 {
      anim = GetComponent("Animator");
 }
 
 function Update () 
 {
      anim.SetFloat("speed",Mathf.Abs(Input.GetAxis("Horizontal")));
      transform.position.x += Input.GetAxis("Horizontal") * movespeed * Time.deltaTime;
 
      if(Input.GetKeyDown(KeyCode.Space) && canjump)
      {
           rigidbody2D.velocity.y = jump;
 
           if(transform.position.y > maxheight)
           {
                transform.position.y = maxheight;
           }
 
           anim.SetTrigger("jump");
      }
 }
 
 function OnCollisionEnter2D(Collision2D collider)
 {
      // You will most likely need to change this
      // This is VERY basic, just saying if we collided
      // with something that is below us
      if(collider.colliderInfo[0].normal.y == 1)
      {
           canjump = true;
      }
 }
 
 function OnCollisionExit2D()
 {
      canjump = false;
 } 

Some tips (this is all meant to be helpful, so in the future you'll get quicker responses):

• Use proper indentation.

• Always use brackets, especially if you do not intend on using proper indentation. Starting on Line 21, it's very hard to read what's going on and what's intended. What you have posted is actually this:

  void Update() { // Your other code here... // ...

    if(Input.GetKeyDown(KeyCode.Space))
      {
         rigidbody2D.velocity.y = jump;
      }
      if(transform.position.y > maxhighet)
      {
         transform.position.y = maxhighet;
      }      
      anim.SetTrigger("jump");
   }