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There was a code posted in the crowd today


      
    
       
  1. let foo = {};
    2. 
       
  2. foo[foo['red'] = 1] = 'red';
    3. 
      
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And exclaimed:

Only to find that JS can also be written like this.

I do not know why he is so feeling, then asked a question, his answer is:

I didn’t know assignment had a return value.

Get back to me with this and I’ll have a rough idea. He may have read second-hand documents, or he may not have studied systematically.

Because in formal documentation, the word assignment “operation” is not used. Instead, expressions and statements are used.

Assignment “expressions” all return a value (any expression returns a value), but assignment “statements” do not.

B =5 is an assignment expression that returns 5, so we can write:

                            
      
    
                                
       
  1. 
                                    
    var  a =  (b =  5);  // ok 
                                                        
                                    
    
                                
    2. 
                            
      

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But varb=5 is an assignment, so we can’t write:

                                
      
    
                                    
       
  1. 
                                        
    var  a =  (var b  = 5 ); // error 
                                                            
                                        
    
                                    
    2. 
                                
      

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Throws an exception: UncaughtSyntaxError: Unexpectedtokenvar

Similarly, if is a statement, so we cannot write:

                                        
      
    
                                            
       
  1. 
                                                
    var  a =  if(true ) {  } 
                                                                    
                                                
    
                                            
    2. 
                                        
      

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Is also very similar: UncaughtSyntaxError: Unexpectedtokenif

You might think: of course I know if can’t be assigned to a variable!!

But ~ ~ ~ ~

In some languages, especially functional programming languages, if-else is also a statement. The selling point of many functional programming languages is that they have expressions and no statements.

For example, in Kotlin, you could write:


      
    
       
  1. val max = if (a > b) a else b
    2. 
      
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Because its if(a>b)aelseb is an expression that returns a value, it can be assigned directly to Max.