0

This question was asked before: Trouble understanding what happens during javascript for loop but the asker didn't get the right answer and so did I. I learn C++ and Python earlier so I get familiar with loop but when I practing this code:

let text = "";
let i;
for(i=0; i<5; i++)
{text += "The number is " + i + " ";}
document.write(text);

the output is totally different to what I expected. I visited W3schools but I can't find the answer https://www.w3schools.com/js/js_loop_for.asp

https://www.w3schools.com/js/tryit.asp?filename=tryjs_loop_for_om3

I expected the output is "" because text can't be changed but the code prints out: "The number is 0 The number is 1 The number is 2 The number is 3 The number is 4". My question is why the print executes in the loop though the print is outside the loop? Thank you very much!

  • 1
    You don't print in the loop. You keep adding more content to the variable called `text` and you print only once, after the loop has finished – malarres Jan 18 '22 at 07:51
  • Thanks for answering but why text is not empty string since it was initialized as text = ""? –  Jan 18 '22 at 07:55
  • 1
    the loop will completely execute before your document.write, so all your concatinations will apply – john Smith Jan 18 '22 at 08:02
  • I totally forgot that text was reassigned. I was confused with the context string is immutable, can't replace characters in it. Thank you! –  Jan 18 '22 at 08:22

2 Answers2

3

The print executes as you expect: Only once at the end. In the loop, each time an extra part is appended to text. That is:

[iteration 0] ""
[iteration 1] "The number is 0 "
[iteration 2] "The number is 0 The number is 1"
[iteration 3] "The number is 0 The number is 1 The number is 2"
etc.

Then in the end, you print the final String which is the concatenation of all the partial Strings you 'glued' together.

Saying x += y is the same as saying x = x + y. Standard variables (var and let) are still mutable, so if you initialize it as an empty String, you can still append/replace it later.

Try making text a const instead, and there should be a runtime TypeError because you try to reassign a constant.

fravolt
  • 2,565
  • 1
  • 4
  • 19
  • How about "Javascript strings are immutable"? I print the text variable, and it should be empty string, why it prints out the string that was added through the loop? –  Jan 18 '22 at 08:00
  • I totally forgot that text was reassigned. I was confused with the context string is immutable, can't replace characters in it. Thank you! –  Jan 18 '22 at 08:24
0

why the output not empty string because the text variable not empty anymore after for loop. you have assign string when loop so the text variable not empty anymore

Ilham Naufal
  • 61
  • 3
  • Thank you for pointing it out! I totally forgot that text was reassigned. I was confused with the context string is immutable, can't replace characters in it. Thank you! –  Jan 18 '22 at 08:21