The following code snippet is supposed to increase the opacity of a certain image(its id is 'img') by 0.01 per every 10 milliseconds until its opacity is set to 1.

    ```var t;
    
    var frontimage=document.getElementById('img'); 
 
    frontimage.style.top=200 'px';
    frontimage.style.opacity=0;                                     //line x
    frontimage.style.left=200 'px';
    frontimage.style.width=500 'px';
    frontimage.style.height=300 'px';

    function right(){
       
     if(frontimage.style.opacity>=0 && frontimage.style.opacity<1){ //condition
        frontimage.style.opacity=frontimage.style.opacity 0.01;     //first statement
        console.log(frontimage.style.opacity);
      
     }else{
        window.clearInterval(t);
        
     }
    }

 t=  window.setInterval(right,10);```

I believe that the 1st statement after the if condition(lets just call it as the first statement for now) gets executed only once while the other statement in the same condition gets executed perfectly as long as the condition remains true. The first statement is supposed to increase the opacity of the image by 0.01. The reason why I say that the first statement gets executed only once is because the 2nd statement of the same condition, which is supposed to display the opacity of the image on the console always displays 0.01(the opacity had been set to 0 before). Could someone please explain to me as to why the first statement gets executed only once when it is supposed to get executed until the condition remains true?

FYI the following code snippet has been created by modifying the 'line x', 'if condition' and the 'first statement' of the above code snippet and is supposed to decrease the opacity of the same image by 0.01 per every 10 milliseconds. This code gets executed perfectly.

        var frontimage=document.getElementById('img');
     
        frontimage.style.top=200 'px';
        frontimage.style.opacity=1;
        frontimage.style.left=200 'px';
        frontimage.style.width=500 'px';
        frontimage.style.height=300 'px';

        function right(){
           
         if(frontimage.style.opacity>0 && frontimage.style.opacity<=1){
            frontimage.style.opacity=frontimage.style.opacity-0.01;
            console.log(frontimage.style.opacity);
          
         }else{
            window.clearInterval(t);
        
         }
        }

     t=  window.setInterval(right,10);

CodePudding user response：

The opacity property will return a string so you need to convert that to a number before you add to it because the operator in JavaScript can mean numeric addition (when both operands are numbers) or string concatenation (when one or both operands are strings).

The reason it works with subtraction is because the - only works with numbers and so any operands that aren't numeric will automatically be converted to a number.

Also (FYI) instead of concatenating your pixel sizes with the px to produce a string, just pass both, together, as a string:

Instead of 200 "px", just use "200px".

var t;
var frontimage = document.getElementById('img'); 
 
frontimage.style.top = "200px";
frontimage.style.opacity = 0;                                    
frontimage.style.left = "200px";
frontimage.style.width = "500px";
frontimage.style.height = "300px";

console.log(typeof frontimage.style.opacity); // string

function right(){
  if(frontimage.style.opacity>=0 && frontimage.style.opacity<1){ 
     frontimage.style.opacity = parseFloat(frontimage.style.opacity)   0.01;     
     //console.log(frontimage.style.opacity);
  }else{
     clearInterval(t);
  }
}
t = setInterval(right,10);

<img src="https://www.drawingforall.net/wp-content/uploads/2020/11/6-how-to-draw-a-smiley-face.jpg.webp" id="img">

CodePudding user response：

• SO in your code every time opacity was getting set to 0 that's your code was keep going in an infinite loop.
• I have used one temporary counter variable and compared it and increased it by 0.01 and updated it and in the end assigned its value to image.style.opacity.
• Moreover, you were comparing string value with number. as it was not with the type check but you should also keep that thing in mind.

let timeIntervalId = null;
const image = document.querySelector("#img");

image.style.top = 200   'px';
image.style.left = 200   'px';
image.style.width = 500   'px';
image.style.height = 300   'px';
image.style.opacity = 0;
var counter = 0;

function right() {
  if (counter >= 0 && counter <= 1) { //condition
    counter = counter   0.01;
    image.style.opacity = counter;
    //first statement
    console.log(image.style.opacity);
  } else {
    window.clearInterval(timeIntervalId);
  }
}

timeIntervalId = window.setInterval(right, 10);

<img id="img" src="https://cdn.pixabay.com/photo/2022/06/23/09/46/mountain-7279430__480.jpg" />

CodePudding user response：

In the first case (the addition), the opacity property and 0.01 frontimage.style.opacity 0.01 are being concatenated rather than added. This is because the opacity property is provided as a string. In the second case (the subtraction), the opacity property is being cast to a number before the arithmetic operation is performed. Do the following typeof (frontimage.style.opacity) so you can see that what it is actually providing you is a string.You have two options

1. parseFloat(frontimage.style.opacity) 0.01
2. frontimage.style.opacity 0.01