Using if/else statement in Google Earth Engine JavaScript API

Question

I am studying because I want to publish my results through Google Earth Engine. Regarding the usage of if / else and ee.Algorithms.If(), I don't understand why both true case and false case work.

As per Code, I want to check if the value selected from ui.Select scorresponds to List v.

var s = ui.Select({
  
  items: [
    
      {label: 'A', value: 'A'},
      {label: 'B', value: 'B'},
    
    ]
  
});

var b = ui.Button('Go!');
b.onClick(b_function);

//var v = ee.List(['A', 'B', 'C']);
var v = ee.List(['C']);

function b_function(){
  
  ee.Algorithms.If(v.contains(s.getValue()), print('True!'), ui.root.add(ui.Label({value:'False!'})));

}

ui.root.add(s);
ui.root.add(b);

Is this possible even with a general if / else statement in google earth engine?

function b_function(){
  
  If(v.contains(s.getValue()){print('True!')}
  else(ui.root.add(ui.Label({'False!'});
  
}

Answer 1

The issue here is a mix up between client- and server-side. I would suggest that you read up on it here.

Your condition is server-side, so you cannot use a regular if statement. On the other hand, your actual actions are both client-side.

All arguments of ee.Algorithms.If() will be evaluated immediately, and both are performing client-side operations before the condition has even been evaluated. The common use of ee.Algorithms.If() is to use the return value, which will be different based on the condition. This server-side return value, ee.String in the example below, can then be turned into a client-side value using evaluate():

  ee.Algorithms.If(
    v.contains(s.getValue()), 
    'True!', 
    'False!'
  ).evaluate(function (value) {
    ui.root.add(ui.Label({value: value}))
  })

But you can just as well turn your condition into a client side object and use client-side conditional logic:

  v.contains(s.getValue()).evaluate(function (contains) {
    if (contains) {
      print('True!')
    } else {
      ui.root.add(ui.Label({value: 'False!'}))
    }
  })

https://code.earthengine.google.com/ff0b00c827899ebc44921cb7e2929c5a