What is the reason for the code to execute without errors using the 'let' keyword, but encountering issues with the 'var' keyword?

Take a look at the code snippet below:

<button id="btn-0">Button 1!</button>
<button id="btn-1">Button 2!</button>
<button id="btn-2">Button 3!</button>
<button id="btn-3">Button 4!</button>

<script type="text/javascript">
    var prizes = ['Watch','A Unicorn!', 'A Hug!', 'Fresh Laundry!'];
    for (var btnNum = 0; btnNum < prizes.length-1; btnNum++) {
        // when each button is clicked...
        document.getElementById('btn-' + btnNum).onclick = function() {
            // display what the user has won!
            alert(prizes[btnNum]);
        };
    }
</script>

In this scenario, using 'var' to declare the variable btnNum in the 'for' loop causes issues. Clicking on any button will result in the same value being alerted, which is the last element of the 'prizes' array.

When 'let' is used instead of 'var', everything functions correctly. Each button now displays a different array element upon clicking. So, why does replacing 'var' with 'let' fix the problem?

<button id="btn-0">Button 1!</button>
<button id="btn-1">Button 2!</button>
<button id="btn-2">Button 3!</button>
<button id="btn-3">Button 4!</button>

<script type="text/javascript">
    var prizes = ['Watch','A Unicorn!', 'A Hug!', 'Fresh Laundry!'];
    for (let btnNum = 0; btnNum < prizes.length-1; btnNum++) {
        // for each button click event...
        document.getElementById('btn-' + btnNum).onclick = function() {
            // show the corresponding prize!
            alert(prizes[btnNum]);
        };
    }
</script>

Answer №1

var declares a function-level variable that is shared by all instances of the loop. The value it holds during each iteration is what subsequent operations will see, as it retains its state after exiting the loop.

let introduces a block-level variable where a new instance is created on every iteration of the loop. This specific variable exists solely within that particular traversal of the loop.

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