效果：
*
***
*****
***
*
for (var i = 0 ; i <= 2; i++) {
for (var j = 0 ; j < 3-i; j++) {
document.write(" ")
}
for (k = 0 ; k < 2*i+1; k++) {
document.write("*")
}
document.write("
")}
for (var i = 0 ; i < 3 ; i++) {
for(var j = 0 ; j <= i ; j++){
document.write(" ")
}
for ( var k = 0; k < 5-i*2 ; k ++) {
document.write("*")
}
document.write("
")}
