Top-down movement with strafing

//// Create event
dir = direction;
strafe = 0;
spd = 4;
image_speed = 0;

//// Step event
var up = keyboard_check(ord('W'));
var left = keyboard_check(ord('A'));
var down = keyboard_check(ord('S'));
var right = keyboard_check(ord('D'));

image_angle = point_direction(x,y,mouse_x,mouse_y);
direction = image_angle;
image_index = 0;

if (up){
    direction = image_angle;
    speed = spd;
}
if (down){
    direction = image_angle+180;
    speed = spd;
}
if (left){
    if (strafe = 1){
        strafe = 0;
    }
    if (strafe = 0){
        dir = image_angle+90;
        strafe = -1;
    }
    direction = dir;
    speed = spd;
}
else if (right){
    if (strafe = -1){
        strafe = 0;
    }
    if (strafe = 0){
        dir = image_angle-90;
        strafe = 1;
    }
    direction = dir;
    speed = spd;
}
if ((!left && !right) || (left && right)){
    strafe = 0;
}
if ((!up && !down && !left && !right) || (left && right) || (up && down)){
    speed = 0;
}