A few examples to figure out how JS's settimeout works

function test() {  var a = 1;  setTimeout(function() {    alert(a);    a = 5;  }, 1000);}test();alert(0);//前面的两个setTimeout延迟了 所以先执行这个

Result: Pop 0 first, then pop up 1.

Example 2

function test() {  var a = 1;  setTimeout(function() {    alert(a);    a = 5;  }, 1000);  lert(a);}test();alert(0);

Result: pops up 1 First, then pops 0, and finally pops up 1.

The alert () in test () is executed first because the function is not executed at this time, so a is still 1. Because the function is delayed, the alert (0) is executed, and the last time, the alert (a) is executed, because the previous a has become 1, so , and the last popup is 1.

Example: 3:

function test() {  var a = 1;  setTimeout(function() {    alert(a);    a = 5;  }, 1000);  a = 19;}test();alert(0);//前面的alert因为setTimeout延迟了 所以先执行这个。

Result: The 0 will pop up first and then pop up 19.

Deferred function () {
alert (a);
A = 5;
}

But the a=19 will still execute, not wait until the end of the delay. So, when executing this delayed function, a=19!

Example 4

function test() {   var a = 1;  setTimeout(function() {  　　alert(a);  　　a = 5;  }, 1000);  a = 19;  setTimeout(function() {    alert(a);    a = 4;  }, 3000); }test();alert(0);//前面的两个setTimeout延迟了 所以先执行这个

Result: Will pop up 0 first, then pop up 19, and finally pop up 5.

Additional explanations for global variables and local variables!

Global variables: Defined outside a function, or assigned within a function but not preceded by the VAR keyword, are global variables

Local variables: variables defined within a function and preceded by a var keyword

A few examples to figure out how JS's settimeout works