Why is the output showing as abnormal?

Question

Why is the output showing as abnormal?

There are occasions when I am unable to successfully execute this script!

var b = [9,8,7,6,5,4,3,2,1]; 
var a = (function(){ 
  var W = []; 
  for(var k in b){ 
    W[W.length] = { 
      index : k, 
      fx : function(){ 
        console.log(k); 
      } 
    } 
  } 
  return W; 
})(); 

console.log(a); 
for(var j = 0; a[j]; j++)a[j].fx();

Why is it not displaying the numerical sequence 987654321 as intended?

javascript

Answer 1

Answer №1

Each function you create called fx references the variable k. Instead of saving the current value of k, they only hold a reference to it. As a result, when you execute these functions after the for loop completes, the final value of k is 8, and all functions will output that value.

To prevent this issue and ensure each function has its own variable/value, you can switch from using var k to let k (which is available starting from ECMAScript 6). For more information on let, you can refer to MDN's documentation on let.

var b = [9,8,7,6,5,4,3,2,1]; 
var a = (function(){ 
  var W = []; 
  for (let k in b) { 
    W[W.length] = { 
      index : k, 
      fx : function(){ 
        console.log(k); 
      } 
    } 
  } 
  return W; 
})(); 
 
for(var j = 0; a[j]; j++)
  a[j].fx();

Answer 2

Each function you create called fx references the variable k. Instead of saving the current value of k, they only hold a reference to it. As a result, when you execute these functions after the for loop completes, the final value of k is 8, and all functions will output that value.

To prevent this issue and ensure each function has its own variable/value, you can switch from using var k to let k (which is available starting from ECMAScript 6). For more information on let, you can refer to MDN's documentation on let.

var b = [9,8,7,6,5,4,3,2,1]; 
var a = (function(){ 
  var W = []; 
  for (let k in b) { 
    W[W.length] = { 
      index : k, 
      fx : function(){ 
        console.log(k); 
      } 
    } 
  } 
  return W; 
})(); 
 
for(var j = 0; a[j]; j++)
  a[j].fx();

Why is the output showing as abnormal?

Answer №1

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