Chicken first or egg？

Let’s look at the following code:

<script>foo();//execute function foofunction foo(){console.log("execute function foo");}console.log(a);//undefined var a=1;
</script>

Can you see the difference?Why they lead to different results?
In fact,function and variable declaration executed during the compilation before runtime.
The code above will execute like this:

<script>function foo(){console.log("execute function foo");}var a;foo();//execute function fooconsole.log(a);//undefined a=1;
</script>

Each declaration will execute first in its own scope before run.

<script>foo();function foo(){console.log(a);//undefinedvar a=1;}
</script>

However，function expression does not work like function declaration.

<script>foo();//TypeError: foo is not a functionvar foo=function(){console.log("foo()");}
</script>

It likes

<script>foo();//TypeError: foo is not a functionvar foo;
</script>

Tips:If you write more than one same function,the behind covers the front.

<script>foo();//2function foo(){console.log("1");}function foo(){console.log("2");}
</script>

Q:fill in the blanks with the correct answers:

<script>var foo=function(){console.log("1");}foo();//①var foo=function(){console.log("2");}foo();//②
</script>

<script>function foo(){console.log("1");}foo();//③var foo=function(){console.log("2");}foo();//④function foo(){console.log("3");}foo();//⑤
</script>

A: [1,2,3,2,2].

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