Javascript movement delay on initial keypress (3D)

I want the movement to be smooth upon pressing a key, the object does an initial movement and then starts moving completely when holding down the button.

//movement speed
var xSpeed = 0.5;
var zSpeed = 0.5;

document.addEventListener("keydown", onDocumentKeyDown, false);

function onDocumentKeyDown(event) {
  var keyCode = event.which;
  if (keyCode == 87) {  //W KEY
    car.position.z -= zSpeed;
  } else if (keyCode == 83) {  //S KEY
    car.position.z += zSpeed;
  } else if (keyCode == 65) {  //A KEY
    car.position.x -= xSpeed;
  } else if (keyCode == 68) {  //D KEY
    car.position.x += xSpeed;
  } else if (keyCode == 32) {
    car.position.set(0, 0, 0);
  }
};

Trying to get a GLTF imported model to be controlled on a plane with keyboard controls smoothly. Tried wrapping the program in a loop and made the state of the keyboard available inside the scope of that loop but couldn't get it working

You could try an approach where the velocity is modified upon keyup/keydown, and the loop just modifies the car's position based on the velocity.

// initial movement speed
var xSpeed = 0;
var zSpeed = 0;

document.addEventListener("keydown", onDocumentKeyDown, false);
document.addEventListener("keydown", onDocumentKeyUp, false);

function onDocumentKeyDown(event) {
  var keyCode = event.which;
  if (keyCode == 87) {  //W KEY
    zSpeed = -0.5;
  } ... // do so for the rest of the keys
};


function onDocumentKeyUp(event) {
  var keyCode = event.which;
  if (keyCode == 87) {  //W KEY
    zSpeed = 0;
  } ... // do so for the rest of the keys
};

function loop() {
car.position.z += zSpeed;
car.position.x += xSpeed;
window.requestAnimationFrame(loop);
}

If you are interested in more eased movement, e.g. the car doesn't just start moving at constant velocity but ramps up and slows down, consider having the keyup/keydown modify an acceleration factor, and the loop modifies the velocity.

This is a good in-depth article by Daniel Shiffman about attempting to represent the real world with code: https://natureofcode.com/book/chapter-1-vectors/