Variable not hoisted | 易学教程

问题

In Javascript variables are hoisted to the top of the scope they are declared within.
However in the following code it seems that the variable myvar is not hoisted.

<html>
<body>
</body>
</html>

<script type="text/javascript">
    console.log(typeof myvar);  

    var myvar = "value";    

    console.log(typeof myvar);  

</script>

The output of the above is:

undefined
string

I expected that the first line would say "string" because myvar is supposed to be hoisted to above it.

Why isn't this so?

回答1:

Variable declarations are hoisted, the assignments are not.

By using var myvar anywhere in the function you create a locally scoped variable myvar, but if you follow it with = something then the assignment will take place in normal code order.

回答2:

A dupe of this came up, and Quentin's answer is good (as always), but just to spell it out even more:

The way the JavaScript engine actually processes that code is like this:

// Before any step-by-step code
var myvar;
// Now step-by-step starts
console.log(typeof myvar);  // undefined since no value has been written to it
myvar = "value";    
console.log(typeof myvar);  // "value" because that's its value now

That is, this:

var myvar = "value";

... is actually two separate things that happen at two separate times. The variable declaration (the var myvar part), which happens before any step-by-step code in the scope is run, and the initialization (the myvar = "value" part; it's really an assignment expression by the time it's processed), which happens where that line is in the code.

来源：https://stackoverflow.com/questions/27770845/variable-not-hoisted

标签

javascript

hoisting