For example, would this:
while (true) {
var random = Math.random();
}
... be less efficient than the following, in most implementations?
var random;
while (true) {
random = Math.random();
}
Thanks for your input.
Edit: In case it wasn't obvious, I'm mostly worried about lots of repeated (de)allocations occurring in this example.
JavaScript does not have block scoping.
In the first example, the var text declaration is hoisted out of the while block. In both cases, the variable is declared only once. In both cases, the variable is assigned a value once per iteration of the while loop.
varfunction-scoped
var random; var interval = setInterval(foo, 100); function foo() { random = Math.random(); is actually faster than if the var was moved inside of foo?
var outside, but there are many other considerations. For example, the browser might perform different optimizations with the declaration inside rather than outside. Something else that might affect the speed is the variable lookup inside of foo.
let and const.No, variables are initiated upon entry into the scope, so random exists before the var statement is even reached.
var statement multiple times in the same scope does nothing in JavaScript than if the var is done outside the loop.while loop" rather than "the body of the function."JavaScript doesn't have block scope, and random's declaration would be hoisted to the top of its scope anyway (variable object).
This depends on the implementation of the interpreter. Strictly speaking, yes, it may have a slightly higher overhead; but depending on the GC mechanism this should be cleared reasonably quickly too.
Douglas Crockford recommends putting all the var assignments at the top of a function, i.e. outwith any loops.
