IF ELSE statement for expression using Google Earth Engine

Question

Code link: https://code.earthengine.google.com/a2c5293fff598d37ccb7843bff8bc27d

In this expression, the value of 'E' is derived from the image. All other values come from the asset table. So I wanted to use if/else in this code:

If 'E' is greater than 0.78, it uses 'A', 'B', 'C', 'D' value defined in expression. Else if it is smaller than 0.78, it uses different 'A', 'B', 'C', 'D' value.

I created the expression code but don't now how to use if/else statement for the expression.

var aqi = image.expression
('((A-B)/(C-D))*(E-D)+B'
, { 
  'A': ee.Number.parse(table.filter(ee.Filter.eq('No2', 10)).first().get('No2')),
 'B': ee.Number.parse(table.filter(ee.Filter.eq('No2', 40)).first().get('No2')),
  'C':ee.Number.parse(table.filter(ee.Filter.eq('No2', 50)).first().get('No2')),
  'D':ee.Number.parse(table.filter(ee.Filter.eq('No2', 80)).first().get('No2')),
  'E':image.select('NO2_column_number_density').multiply(1000),

Answer 1

You don't do it with an if, you do it by computing both and selecting one vs the other with a where() statement.

result1 = image.expression('(A1-B1) / (C1-D1)...')
result2 = image2.expression('(A2 - B2) / (C2-D2)...')
result = result1.where(e.lt(0.78), result2)