JavaScript closure inside loops – simple practical example
var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
// and store them in funcs
funcs[i] = function() {
// each should log its value.
console.log("My value: " + i);
};
}
for (var j = 0; j < 3; j++) {
// and now let's run each one to see
funcs[j]();
}It outputs this:
My value: 3
My value: 3
My value: 3
Whereas I'd like it to output:
My value: 0
My value: 1
My value: 2
The same problem occurs when the delay in running the function is caused by using event listeners:
var buttons = document.getElementsByTagName("button");
// let's create 3 functions
for (var i = 0; i < buttons.length; i++) {
// as event listeners
buttons[i].addEventListener("click", function() {
// each should log its value.
console.log("My value: " + i);
});
}<button>0</button>
<br />
<button>1</button>
<br />
<button>2</button>… or asynchronous code, e.g. using Promises:
// Some async wait function
const wait = (ms) => new Promise((resolve, reject) => setTimeout(resolve, ms));
for (var i = 0; i < 3; i++) {
// Log `i` as soon as each promise resolves.
wait(i * 100).then(() => console.log(i));
}It is also apparent in for in and for of loops:
const arr = [1,2,3];
const fns = [];
for(var i in arr){
fns.push(() => console.log(`index: ${i}`));
}
for(var v of arr){
fns.push(() => console.log(`value: ${v}`));
}
for(var f of fns){
f();
}What’s the solution to this basic problem?