javascript function vs var function

The difference is that start is a function expression and so only defined when that line is reached, whereas M is a function declaration and is defined as soon as its surrounding function or script is executed.

For example, a function expression:

// TypeError: undefined is not a function

start();
var start=function(){
    console.log("Start function called !");
}

//It will work 
m();
 function m() {
    console.log("Function M is working fine");
   
}