What does "+=" mean? I'm working with an esp8266 and want to have it default to an access point when it cannot connect to a wifi network.  I'm using chriscook8's example from github here.

chriscook8 uses "+=" in several places and I can't find an explanation of what that means.

Here is an example from setup():

void setup() {
  Serial.begin(115200);
  EEPROM.begin(512);
  delay(10);
  Serial.println();
  Serial.println();
  Serial.println("Startup");
  // read eeprom for ssid and pass
  Serial.println("Reading EEPROM ssid");
  String esid;
  for (int i = 0; i &lt; 32; ++i)
  {
    esid += char(EEPROM.read(i));  //&lt;- here
  }


What does += do?
It adds the value or variable on the right to the variable on the left and assigns the result to the variable on the left.

A += B


is the same as:

A = A + B


It's short-hand.

In the case of a String it concatenates the right hand item on to the end of the left hand item.