Why is this for loop using +=?

Question

I'm having trouble understanding why this for loop is text += cars[i]... and not just text = cars [i]...

Here is the full script

var cars = ["BMW", "Volvo", "Saab", "Ford"];
var i, len, text;
for (i = 0, len = cars.length, text = ""; i < len; i++) {
  text += cars[i] + "<br>";
}

document.getElementById("demo").innerHTML = text;

<p id="demo"></p>

Try it and find out. If it was `text = ...`, then `text` would only reflect the final iteration. — CertainPerformance, Aug 02 '18 at 01:07
not sure which part of `+=` confuses you ... `text += cars` .... is `text = text + cars` ... and `'string1' + 'string2'` results in `string1string2` - — Jaromanda X, Aug 02 '18 at 01:11
@JaromandaX, believe you have best comment. Consider making it an answer so can be accepted? — asantaballa, Aug 02 '18 at 01:16
@asantaballa - I don't believe in posting answers for what I consider to be `Javascript 101` type questions - a javascript programmer should know what `+=` is within the first hour of their javascript programming journey :p — Jaromanda X, Aug 02 '18 at 01:19
@JaromandaX, understand and hope did not offend. Just see many people lose credit when without realizing it when they in fact answered the question. Thanks for response. — asantaballa, Aug 02 '18 at 01:22

score 1 · Answer 1 · answered Aug 02 '18 at 01:17

1

= is assignment

+= is assignment and addition

so if text += 'abc'

and text was initially def,

the result will be defabc

in your case, the result would be

BMW<br/>Volvo</br>Saab<br/>Ford<br/>

answered Aug 02 '18 at 01:17

kkesley

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score 0 · Answer 2 · answered Aug 02 '18 at 01:15

The += operator, such as: x += y, is the same thing as writing x = x + y. In your example, each item in the cars array is being added to a new variable, text, with a 'br' tag inserted between them. It should list all of the cars listed in your array one above the other. If you just used the = operator, the 'text' variable would be overwritten several times until the loop ends, thus you would just see the entry 'Ford' as it's the last in the array.

score 0 · Answer 3 · answered Aug 02 '18 at 01:25

using += the output is BMW Volvo Saab Ford

So basically since its a for loop , on each iteration the value of text is built as BMW
Volvo
Saab
Ford

If you remove the + from the script on each iteration the value of text will be replaced by the car[i] and the end value of the variable text will be Ford

Why is this for loop using +=?

3 Answers3