Why do we need to add the var keyword when declaring variables in JavaScript_javascript tips

In JavaScript, var is used to declare variables, but this syntax is not strictly required. In many cases, we can use a variable directly without declaring it with var.

The code is as follows:

window. y = "hello"; 
function func(){ 
y = "OH, NO!!!"; 
} 
func(); 
alert(window.y); //#=> display "OH, NO!!!" 

When any layer in the context has such an "implicitly" defined variable, the variable in that layer will be modified without generating a new variable on the window. (This kind of bug is also quite annoying, especially when encapsulating more complex code)
For example:

The code is as follows:

var x = "window.x"; 
function a() { 
var x = "a's x"; 
var b = function() { 
var c = function() { 
//no var! 
x = "c's x:"; 
}; 
alert("before c run,the b.x:" + x); 
c(); 
alert("after c run, the b.x:" + x); 
}; 
alert("a.x is:" + x); 
b(); 
alert("after b function runed, the a.x is:" + x); 
}; 
alert("before a run, window.x:" + x); 
a(); 
alert("after a run, window.x:" + x); 

There are the following layers: window, func a, func b, func c are always hierarchically nested. window->a->b->c
In both window and a, the variable x is defined, but in b the variable is not defined. In c, an x ​​is declared 'implicitly', and the x ultimately modifies the value of the a variable.
Remember, in JavaScript, when declaring a variable, it must be preceded by var.