How do I build a loop in JavaScript? [closed]
-
09-06-2019 - |
Question
How can I build a loop in JavaScript?
Solution
For loops
for (i = startValue; i <= endValue; i++) {
// Before the loop: i is set to startValue
// After each iteration of the loop: i++ is executed
// The loop continues as long as i <= endValue is true
}
For...in loops
for (i in things) {
// If things is an array, i will usually contain the array keys *not advised*
// If things is an object, i will contain the member names
// Either way, access values using: things[i]
}
It is bad practice to use for...in
loops to itterate over arrays. It goes against the ECMA 262 standard and can cause problems when non-standard attributes or methods are added to the Array object, e.g. by Prototype.
(Thanks to Chase Seibert for pointing this out in the comments)
While loops
while (myCondition) {
// The loop will continue until myCondition is false
}
OTHER TIPS
Here is an example of a for loop:
We have an array of items nodes.
for(var i = 0; i< nodes.length; i++){
var node = nodes[i];
alert(node);
}
You might also consider optimizing your loop speed; see http://www.robertnyman.com/2008/04/11/javascript-loop-performance/
Aside form the build-in loops (while() ...
, do ... while()
, for() ...
), there is a structure of self calling function, also known as recursion to create a loop without the three build-in loop structures.
Consider the following:
// set the initial value
var loopCounter = 3;
// the body of the loop
function loop() {
// this is only to show something, done in the loop
document.write(loopCounter + '<br>');
// decrease the loopCounter, to prevent running forever
loopCounter--;
// test loopCounter and if truthy call loop() again
loopCounter && loop();
}
// invoke the loop
loop();
Needless to say that this structure is often used in combination with a return value, so this is a small example how to deal with value that is not in the first time available, but at the end of the recursion:
function f(n) {
// return values for 3 to 1
// n -n ~-n !~-n +!~-n return
// conv int neg bitnot not number
// 3 -3 2 false 0 3 * f(2)
// 2 -2 1 false 0 2 * f(1)
// 1 -1 0 true 1 1
// so it takes a positive integer and do some conversion like changed sign, apply
// bitwise not, do logical not and cast it to number. if this value is then
// truthy, then return the value. if not, then return the product of the given
// value and the return value of the call with the decreased number
return +!~-n || n * f(n - 1);
}
document.write(f(7));
A loop in JavaScript looks like this:
for (var = startvalue; var <= endvalue; var = var + increment) {
// code to be executed
}