Today I came across the following JS:
for (var b = k, d = function (c) {
// function body
}, a = b.p, e = z; 0 < c;) {
} // for loop closing cause
Can some please explain what's happening here? I'm confused about the function, normal "for" loop would do:
for(x=0; x<something; x++){
}
This loops appears to follow a different structure:
for(x=0; d = function(c), a = somevar, e = somevar, 0 < c)
Why are there 5 elements in the for loop? Thanks!
for (var b = k, d = function (c) {}, a = b.p, e = z; 0 < c;) {
} // for loop closing cause
for(x=0; x<something; x++)
If you map the first for loop with the signature for loop...
The code in bold is the declaration part for different variables that are going to be used inside the for loop.. .. Any number of variables can be declared here...
The second is the check condition...
And the third part is empty
There are not five parts, only three:
b, d, a and e).0 < c).Each part is separated by semicolons (;) and the variable declarations are separated by commas ,.
The first part is allowed to contain either an expression or a variable declaration list. For more information, have a look at the specification or the MDN JavaScript documentation:
initialization
An expression (including assignment expressions) or variable declaration. Typically used to initialize a counter variable. This expression may optionally declare new variables with thevarkeyword. These variables are not local to the loop, i.e. they are in the same scope the for loop is in. The result of this expression is discarded.
Only because for(x=0; x<something; x++) is the most typical form, does not mean others don't exist. Each part can contain an arbitrary expression. For example, a useful way to iterate over all children of a DOM element is the following:
for(var node = element.firstChild; node; node = node.nextSibling) {
}
First, node is initialised with the first child of the element. The second part just tests whether node is not null (you could also write node !== null explicitly) and the third part (executed after the iteration) assigns the next child to node.
Ill explain, but basically this is not good code.
Its not good code because you need to go online just to figure it out.
for (var b = k, d = function (c) { // function body }, a = b.p, e = z; 0 < c;) {
} // for loop closing cause
is the equivalent of
var b = k,
d = function (c) {
// function body
},
a = b.p,
e = z;
for (; 0 < c; ){
}
for loop parents has three parts 1. Initialization 2. Condition 3. Ending statement
That must all be separated by a ;
except these are all optional in a for loop
for (var init = 0; init < 10; init += 1) {}
same as:
var init = 0;
for (;;) {
if (!(init < 10)) {
break;
}
init += 10;
}
The control variables in a loop can also be a valid expressions as you have in your example.
The first loop you posted is slightly different from the last. The first declares and assigns several variables (one of which is a function), then provides a condition (0<c), then does nothing every iteration. The last seems invalid.
The problem with the first one seems to be only that it does not intialize a c, so unless c is a variable from outside of the scope of the loop that is somehow being changed inside its body, the loop will either not run at all (if c>=0) or it will run forever (if c is indeed less than 0).
for (var b = k, d = function (c) {
// function body
}, a = b.p, e = z; 0 < c;) {
} // for loop closing cause
as long as d is not 0 the for loop will run. d is 0 when 0 < c
forloops have three parts:for (initialization; condition; increment). Any of these parts can have zero or more statements, ie:for (;;)is valid and so isfor (a=1,b=2,c=3; a!=10; )