JS function Learning

1 function declarations and function expressions

A function declaration defines a function by naming it, and a function expression is an anonymous way of defining a function (a function is an object that executes code in a particular environment)

function A () {
Console.log (' a ');
}
A (); function declaration

var a = function () {
Console.log (' a ');
};
A ();//function expression

The difference between the two is that the way the function is declared can be promoted, which means that the function is called before the function's declaration statement

A ();
function A () {
Console.log (' a ');
}//This won't be an error.

function expressions cannot be

A ();
var a = function () {
Console.log (' a ');
}; will be error a not defined

2 Arguments objects

function Test () {
Console.log (arguments[0],arguments[1],arguments[2],arguments.length);
}

Test (+/-); 1 2 3 3

Use the arguments object to get the corresponding arguments

There's another way to use it.

function factorial (n) {
if (n <= 1) {
return 1;
} else {
Return N*arguments.callee (n-1);
}
}
Console.log (factorial (5));

Get a reference to the function object being executed by Arguments.callee (very good usage)

3 scopes

var a = 100;

function Test () {
Console.log (a);

var a = 20;
Console.log (a);
}
Test ();//undefined 20

I started to think that the first a would access global variables outside of the function, not really, because the scope of Var a declared inside the function was the entire function and it was not assigned when it was first accessed.

The code above is equivalent to

var a = 100;

function Test () {
var A;
Console.log (a);

var a = 20;
Console.log (a);
}
Test ();

There is no block-level scope in JavaScript, but a let statement can be used to achieve block-level scope effects

var a = 10;
{
let a = 100;
Console.log (a);//100
}
Console.log (a);//10

JS function Learning