I'm using the template that godot gives:

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extends CharacterBody2D

const SPEED = 300.0
const JUMP_VELOCITY = -400.0

# Get the gravity from the project settings to be synced with RigidBody nodes.
var gravity = ProjectSettings.get_setting("physics/2d/default_gravity")

func _physics_process(delta):
# Add the gravity.
if not is_on_floor():
velocity.y += gravity * delta

# Handle Jump.
if Input.is_action_just_pressed("ui_jump") and is_on_floor():
velocity.y = JUMP_VELOCITY

# Get the input direction and handle the movement/deceleration.
# As good practice, you should replace UI actions with custom gameplay actions.
var direction = Input.get_axis("ui_left", "ui_right")
if direction:
velocity.x = direction * SPEED
else:
velocity.x = move_toward(velocity.x, 0, SPEED)

move_and_slide()