An instance of apply

Look at the theoretical knowledge of the Apply function when it is very simple, but later to see the book a small code, found that there is no deep understanding of the function, here write down to make a note.

The code is as follows:

function r (x) {
return x;
}
function f (x) {
X[0] =x[0]+ ">";
return x;
}
function O () {
var temp = R;

　　R = function () {

Return temp.apply (this,f (arguments));

}
}

function A () {
O ();
document.write (r ("="));
}

for (var i = 0; I <10; i++) {

A ();

document.write ("</br>")
}

The results of the operation are as follows:

At first, I did not want to understand why printing = re-printing, and later on the code after the breakpoint processing, only to discover the mystery.

function r (x) {
document.write ("6");
return x;
}
function f (x) {
X[0] =x[0]+ ">";
document.write ("5");
for (var i = 0; i < arguments.length; i++) {
document.write (X[i]);
};
return x;
}
function O () {
var temp = R;

document.write ("1");

R = function () {
document.write ("4");
Return temp.apply (this,f (arguments));

}
document.write ("2");
}

function A () {
document.write (r ("="));
O ();
document.write ("3");
document.write (r ("="));
}

A ();
document.write ("<br><br>");

for (var i = 0; I <10; i++) {
A ();

document.write ("</br>")
// }

Following the instructions above, the code runs in this order:

Run O ()

Assign the R function to temp;

Print 1;

Rewrite R function, do not execute;

Print 2;

Back to a function, print 3;

Execute r ("=") function;

The R function is rewritten in O (), printing 4;

Returns a temp function

The return value. Execute f (arguments) First,

Printing 5

Because the arguments at this time is "=", the result of f ("=") is return = =

Execute function Temp (this, "=")

Printing 6

Back to =

So the final result is 123456=>.

Attention:

Temp = R is an assignment to a function;

R = function () {...} just wrote a function and did not call him, therefore do not execute the function;

If the first argument in the Apply function is this, that is, the function that calls him, if it is a different object or function, executes the object set by the first parameter, the second argument is an arguments array, and if it is a function, arguments is the return value of the function.

An instance of apply