How to cover the Approval Process in test class in Salesforce

# Approach 1

Ex: you have an approval process on an opportunity when the amount is greater than 500 the opportunity requires an approval.
Create the opportunity test data as per the approval process criteria and query the process instance object data and use the assert methods to verify the values.

Sample Code

Opportunity objOpp = new Opportunity(Name = 'Test Opp', amount = 800);
insert objOpp;

ProcessInstance objProcessInstance = [SELECT Id,TargetObjectId, CreatedDate FROM ProcessInstance WHERE TargetObjectId = :objOpp.Id];
System.assertEquals(objOpp.CreatedDate,objProcessInstance.CreatedDate);

# Approach 2

Create the Opportunity test data and submit the Opportunity for the approval process using the ProcessSubmitRequest method.

Sample Code

Opportunity objOpp = new Opportunity(Name = 'Test Opp', amount = 500);
insert objOpp;

Approval.ProcessSubmitRequest app = new Approval.ProcessSubmitRequest();
app.setObjectId(objOpp.id);
Approval.ProcessResult result = Approval.process(app);