html - Difference between "alert(a)'' and ''alert(a);var a =1;'' in jav

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<script type="text/javascript">
    alert(a);
</script>

Console log shows : "Uncaught ReferenceError: a is not defined";

<script type="text/javascript">
    alert(a);
    var a = 1;
</script>

at the middle of the browse, Log shows: "undefined"

How does this code run in js and what causes this difference

<script type="text/javascript">
    alert(a);
</script>

Console log shows : "Uncaught ReferenceError: a is not defined";

<script type="text/javascript">
    alert(a);
    var a = 1;
</script>

at the middle of the browse, Log shows: "undefined"

How does this code run in js and what causes this difference

• javascript
• html

Share Improve this question Follow edited Jan 28, 2016 at 6:51 stark 2,25622 gold badges2424 silver badges3535 bronze badges asked Jan 28, 2016 at 6:43 huihui 60188 silver badges2222 bronze badges 3

• 7 Variable hoisting – Tushar Commented Jan 28, 2016 at 6:44
• in second case var a = 1; if you will declare variable after alert undefined error will occurs – Gautam Jha Commented Jan 28, 2016 at 6:49
• there is a typo it should be alert(a) not alter(a) – brk Commented Jan 28, 2016 at 6:49

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1 Answer 1

Sorted by: Reset to default 7

in this code

<script type="text/javascript">
    alert(a);
    var a = 1;
</script>

var a ; is hoisted to the top and it bees

<script type="text/javascript">
    var a;
    alert(a);
    a = 1;
</script>

so by the time a was alerted, it was undefined

In this code

<script type="text/javascript">
    alert(a);
</script>

a was not defined at all, so it gave an error "Uncaught ReferenceError: a is not defined"

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