Three ways and differences of defining functions in JS

1、 Function declaration （ priority of use , It will be promoted when it is declared ）

function fun(){
    
    console.log('123');
}
fun()// Call the function name directly 

2、 Function expression , Also called anonymous function ;

var fun = function(){
    
    console.log('123');
}
fun()// Call anonymous functions , Use variable names plus （）

The process ： This is defined by assigning a function to a variable , Function has no real name ; Since it's a variable , When making a statement , The function itself is not promoted , What is promoted is the variable name ;

3、 Self executing functions ( Do it yourself , And call yourself at the same time of declaration , It can only be called once )

(function()(
    console.log('123')
})();
// principle ： Evolved from a functional expression 

The process ： stay 2 in , hold fun(), Medium fun Directly replace with the function body , became function(){}(); Plus （） Enclosed function body , lest js False positives are function declarations