Don’t iterate so much

for(var key in obj){
    if(!obj.hasOwnProperty(key)){
        continue;
    }

    // Do something.
}
						

var keys = Object.keys(obj);

for(var i = 0; i < keys.length; i++){
    // Object.keys only returns own properties.
    // Do something.
}
						

Memoisation

function isPrime(n) {
    if (isNaN(n) || !isFinite(n) || n % 1 || n < 2) {
        return false;
    }

    var m = Math.sqrt(n);

    for (var i = 2; i <= m; i++) {
        if (n % i === 0) {
            return false;
        }
    }

    return true;
}
						

var cache = {};
function isPrimeMemoised(n) {
    if(n in cache){
        return cache[n];
    }
    if (isNaN(n) || !isFinite(n) || n % 1 || n < 2) {
        return cache[n] = false;
    }

    var m = Math.sqrt(n);

    for (var i = 2; i <= m; i++) {
        if (n % i === 0) {
            return cache[n] = false;
        }
    }

    return cache[n] = true;
}
						

Results

Good for

Bad for

Impossible for

Reuse objects

function onMouseMove(event){
	$('.someUniqueElement').left(event.pageY);
	$('.someUniqueElement').top(event.pageY);
}
						

var someUniqueElement = $('.someUniqueElement');

function onMouseMove(event){
	someUniqueElement.left(event.pageY);
	someUniqueElement.top(event.pageY);
}