Variables and function declaration advance questions

Experience Summary:

1, the declaration of the variable is advanced to the top of the scope, the assignment is retained in situ;

2, the function declaration is in advance;

3. When the function is assigned to a variable as a value, only the variable is "advanced" and the function is not "advanced".

As a best practice:

Variable declarations must be placed at the top of the scope/function (JavaScript has only function scope).

Example:

//The variable declaration is advanced to the top of the scope and the assignment remains in place. varA = ' out ';(functioninside () {alert (' Before: ' + a);//undefined     varA = ' in '; Alert (' After: ' + a);//' in '} ()) Alert (' Out: ' + a);//' Out '//equivalentvarA = ' out ';(functioninside () {varA//declaration in advanceAlert (' Before: ' + a);//A is not initialized, so it is undefinedA = ' in '; Alert (' After: ' + a);//A is assigned a value of ' in '} ()) Alert (' Out: ' + a);

//the function declaration is in advance//function declaration format: Functions fun () {...}Fun ();//' Hello,world. 'functionFun () {alert (' Hello,world. '));  }fun (); //' Hello,world. '//when the function is assigned to a variable as a value, only the variable is advanced and the function is not advancedFun ();//TypeError error, stating that fun is not a functionvarFun =function() {alert (' Hello,world. '));  };fun (); //' Hello,world. 'equivalent to:varFun;fun (); //Fun hasn't been initialized yet, so it's not a functionFun =function() {alert (' Hello,world. '));  };fun (); //' Hello,world. '

The specific details of this knowledge point is probably so much, seriously think about experience.

Reference:

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Variables and function declaration advance questions