CODEWITHSUNDEEP
javascriptarrays
Tarnz
Tarnz

Reputation: 3

Why does overwriting a whole array within a function not replace it, but overwriting element by element does?

I am learning javascript. One of the exercises is to write a function that overwrites the values in an array passed in as a parameter. My solution worked within the function, but it did not seem to be passed back from the function. Also if I do the same thing that is not in a function, it also works. After I worked out what the problem was I wrote simplified versions of my solution vs a version that works. But I still don't understand why one works and one doesn't. Is someone able to explain?

Thanks

function reverseArrayInPlace(ary){                                
  // This version is successful                                   
  ary[0] = 5;                                                     
  ary[1] = 4;                                                     
  ary[2] = 3;                                                     
  ary[3] = 2;                                                     
  ary[4] = 1;                                                     
  console.log("Within reverseArrayInPlace: " + ary);              
  return ary;                                                     
  }                                                               

function reverseArrayInPlace2(ary){                               
  // This version is not successful                               
  ary = [5, 4, 3, 2, 1];                                          
  console.log("Within reverseArrayInPlace2: " + ary);             
  return ary;                                                     
}                                                                 

var arrayValue = [1, 2, 3, 4, 5];                                 
reverseArrayInPlace(arrayValue);                                  
console.log("After reverseArrayInPlace: " + arrayValue);          

var arrayValue2 = [1, 2, 3, 4, 5];                                
reverseArrayInPlace2(arrayValue2);                                
console.log("After reverseArrayInPlace2 - why is order wrong?: " + arrayValue2);

// But this works.
arrayValue2 = [5, 4, 3, 2, 1];                                    
console.log("After outer replace of arrayValue2: " + arrayValue2);

Results:

Within reverseArrayInPlace: 5,4,3,2,1

After reverseArrayInPlace: 5,4,3,2,1

Within reverseArrayInPlace2: 5,4,3,2,1

After reverseArrayInPlace2 - why is order wrong?: 1,2,3,4,5

After outer replace of arrayValue2: 5,4,3,2,1

Upvotes: 0

Views: 154

Answers (2)

Starfish
Starfish

Reputation: 3574

You're not saving the results.

Do this instead:

var arrayValue = [1, 2, 3, 4, 5];                                 
arrayValue = reverseArrayInPlace(arrayValue);                                  
console.log("After reverseArrayInPlace: " + arrayValue);          

var arrayValue2 = [1, 2, 3, 4, 5];                                
arrayValue2 = reverseArrayInPlace2(arrayValue2);                                
console.log("After reverseArrayInPlace2 - why is order wrong?: " + arrayValue2);

or

var arrayValue = [1, 2, 3, 4, 5];                                                            
console.log("After reverseArrayInPlace: " + reverseArrayInPlace(arrayValue));          

var arrayValue2 = [1, 2, 3, 4, 5];                                
console.log("After reverseArrayInPlace2 - why is order wrong?: " + reverseArrayInPlace2(arrayValue2));

Upvotes: 1

Pointy
Pointy

Reputation: 413916

JavaScript is a pass-by-value (or call-by-value, if you prefer) language. Assigning a new value to a function parameter inside the function has no effect on the outside world. However, using a passed-in reference to an object to modify property values does affect the outside world. You pass in a reference to the array, and then the function can use that reference to stuff new values into the elements of the array.

Your function returns the modified array, but your calling environment ignores that. Change your test:

var arrayValue2 = [1, 2, 3, 4, 5];                                
arrayValue2 = reverseArrayInPlace2(arrayValue2);                                
console.log("After reverseArrayInPlace2 - why is order wrong?: " + arrayValue2);

Upvotes: 3

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