What is the difference between Subject and BehaviorSubject

Question

I m not clear on the difference between a Subject and a BehaviorSubject  Is it just that a BehaviorSubject has the getValue   function

User · Answer

BehaviorSubject keeps in memory the last value that was emitted by the observable  A regular Subject doesn t   BehaviorSubject is like ReplaySubject with a buffer size of 1

User · Answer

A BehaviorSubject holds one value  When it is subscribed it emits the value immediately  A Subject doesn t hold a value  Subject example  with RxJS 5 API   const subject   new Rx Subject    subject next 1   subject subscribe x   gt  console log x     Console output will be empty BehaviorSubject example  const subject   new Rx BehaviorSubject 0   subject next 1   subject subscribe x   gt  console log x     Console output  1 In addition   BehaviorSubject should be created with an initial value  new Rx BehaviorSubject 1  Consider ReplaySubject if you want the subject to hold more than one value

User · Answer

I just created a project which explain what is the difference between all subjects  https   github com piecioshka rxjs-subject-vs-behavior-vs-replay-vs-async

User · Answer

It might help you to understand   import   as Rx from  rxjs    const subject1   new Rx Subject    subject1 next 1   subject1 subscribe x   gt  console log x       will print nothing - gt  because we subscribed after the emission and it does not hold the value   const subject2   new Rx Subject    subject2 subscribe x   gt  console log x       print 1 - gt  because the emission happend after the subscription  subject2 next 1    const behavSubject1   new Rx BehaviorSubject 1   behavSubject1 next 2   behavSubject1 subscribe x   gt  console log x       print 2 - gt  because it holds the value   const behavSubject2   new Rx BehaviorSubject 1   behavSubject2 subscribe x   gt  console log  val    x       print 1 - gt  default value behavSubject2 next 2     just because of next emission will print 2

User · Answer

BehaviourSubject  BehaviourSubject will return the initial value or the current value on Subscription  var bSubject  new Rx BehaviorSubject 0       0 is the initial value  bSubject subscribe     next   v    gt  console log  observerA      v      output initial value  then new values on  next  triggers      bSubject next 1       output new value 1 for  observer A  bSubject next 2       output new value 2 for  observer A   current value 2 for  Observer B  on subscription  bSubject subscribe     next   v    gt  console log  observerB      v      output current value 2  then new values on  next  triggers      bSubject next 3     With output   observerA  0 observerA  1 observerA  2 observerB  2 observerA  3 observerB  3   Subject  Subject does not return the current value on Subscription  It triggers only on  next value  call and return output the value   var subject   new Rx Subject     subject next 1     Subjects will not output this value  subject subscribe     next   v    gt  console log  observerA      v      subject subscribe     next   v    gt  console log  observerB      v       subject next 2   subject next 3     With the following output on the console   observerA  2 observerB  2 observerA  3 observerB  3

[rxjs] What is the difference between Subject and BehaviorSubject?

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