Millis instead of Delay

Projects Programming
1 
// MultiArrays - see https://github.com/FastLED/FastLED/wiki/Multiple-Controller-Examples for more info on
// using multiple controllers.  In this example, we're going to set up four NEOPIXEL strips on three
// different pins, each strip getting its own CRGB array to be played with

#include "FastLED.h"

#define NUM_LEDS_PER_STRIP 11

CRGB redLeds[NUM_LEDS_PER_STRIP];

CRGB blueLeds[NUM_LEDS_PER_STRIP];

unsigned long previousMillis = 0;   
const long Speed = 500;
const long WaitB = 1000;

// For mirroring strips, all the "special" stuff happens just in setup.  We
// just addLeds multiple times, once for each strip
void setup() {
  // tell FastLED there's 60 NEOPIXEL leds on pin 10
  FastLED.addLeds<NEOPIXEL, 12>(redLeds, NUM_LEDS_PER_STRIP);


  // tell FastLED there's 60 NEOPIXEL leds on pin 12
  FastLED.addLeds<NEOPIXEL, 13>(blueLeds, NUM_LEDS_PER_STRIP);

}

void loop() {

  unsigned long currentMillis = millis();

    
    for(int i = 0; i < NUM_LEDS_PER_STRIP; i++) {
   
    // set our current dot to red, green, and blue
    redLeds[i] = CRGB::Red;
    redLeds[i+1] = CRGB::Red;
    blueLeds[i-1] = CRGB::Blue;
    blueLeds[i-2] = CRGB::Blue;
    FastLED.show();
    // clear our current dot before we move on
    redLeds[i] = CRGB::Black;
    redLeds[i+1] = CRGB::Black;
    blueLeds[i-1] = CRGB::Black;
    blueLeds[i-2] = CRGB::Black;
    delay(100);
    
    }
      
    for(int i = NUM_LEDS_PER_STRIP-1; i >= 0; i--) {
    // set our current dot to red, green, and blue
    
    redLeds[i] = CRGB::Black;
    redLeds[i+1] = CRGB::Black;
    blueLeds[i-1] = CRGB::Black;
    blueLeds[i-2] = CRGB::Black;
    FastLED.show();
    redLeds[i] = CRGB::Black;
    redLeds[i+1] = CRGB::Black;
    blueLeds[i-1] = CRGB::Black;
    blueLeds[i-2] = CRGB::Black;
    delay(60);
    }
  }

In this code there are 2 delays and I want to get rid of them because I'm going to implement the code into another code. I have tried to do it but without succes. I get what to do with millis() in the blinkwithoutdelay sketch but I don't get it to work with this code.

2 
  // tell FastLED there's 60 NEOPIXEL leds on pin 10

This comment is wrong. Both specific pieces of information are wrong. Either delete the comment entirely or re-write the comment to make it more descriptive of the end goal, not just repeating the code that it's commenting on.

Your code appears to be two animations. One lights a pair of LEDs on each strip and carries that down the strip, the other just seems to make both strips entirely black, but it takes a lot of time to show a lot of black. If you have two animations, you can make two functions. So the main code can call each animation depending on what it wants to show. Or you can make it into one long animation.

Either way, the animation function needs to remember where it's up to. It needs to record the value of i as static variable, that way i isn't forgotten each time the function exits. Then it needs to record the time that it last advanced i to the next position. Lastly, it may need to reset at the end - you haven't specified if you want the animation to go 'forever' or just once. For a just-once type of animation, the animation function should return true to the main loop when it's finished. Every other time it should return false.