How to better optimize JavaScript code?

I decide to write up some tips for better optimization of JavaScript code. You can use these advices on client-side as well as server-side in Node.js.

1. Array optimization

Repetition is the mother of wisdom and a lot of programmers forget about caching length of the array in loops. This optimization can decrease CPU consuption.

// BAD
var arr = [...];
for (var i = 0; i < arr.length; i++) {

}

// BETTER
var arr = [...];
for (var i = 0, length = arr.length; i < length; i++) {

}

2. Objects caching

I didn't see any similar optimization anywhere and I dare say that this optimization can increase performance and decrease CPU + RAM consuption. In the below example usage in Total.js (Node.js) controller:

// SLOW
function some_action_with_json_response() {
    this.json({ success: true });
}

// MUCH FASTER
const response = { success: true };

function some_action_with_json_response() {
    this.json(response);
}

A similar approach can be applied to Arrays.

3. Assignment of values is faster than creating new objects

This tip follows optimization tip #2. A value assignment is a lot faster than creating a new objects with subsequent assignment. Similar optimization can be used in Express.js or any other Node.js framework.

// SLOW
function some_action_with_json_response() {
    this.json({ success: this.query.success === '1' });
}

// MUCH FASTER
const response = { success: true };

function some_action_with_json_response() {
    response.success = this.query.success === '1';
    this.json(response);
}

SUCCESS() method in Total.js framework uses the same optimization.

4. Do you use "delete" before "JSON.stringify()"?

If yes then I have a good optimization tip, which can increase performance a lot. The keyword delete is helpful, but too slow. Assigning undefined to the property will have the same effect as using delete.

// SLOW
var obj = { name: 'Peter', age: 30 };
delete obj.name;
console.log(JSON.stringify(obj)); // { age: 30 }

// MUCH FASTER
var obj = { name: 'Peter', age: 30 };
obj.name = undefined;
console.log(JSON.stringify(obj)); //{ age: 30 }

5. Date caching also helps

There are cases when there's no need for an accurate time and +/- 1 minute is sufficient. In some cases is not needed an exact time, but it's sufficient +/- 1 minute of difference. I solve this cache via property F.datetime in Total.js framework. This property is refreshed each 1 minut about a new current datetime. So if you don't need exact time on the seconds then you can use my next tip for increasing of perfomance:

// The variable below will have always current datetime in +/- 30 seconds interval.
var datetime = new Date();
setInterval(function() {
    datetime = new Date();
}, 30000);

Real test:

require('total.js')

console.time('With cached Date');
var dt_c = null;
for(var i = 0; i < 1000000; i++) {
    dt_c = F.datetime;
}
console.timeEnd('With cached Date'); // about 5ms

console.time('Without cached Date');
var dt_wc;
for(var i = 0; i < 1000000; i++) {
    dt_wc = new Date();
}
console.timeEnd('Without cached Date'); // about 250ms

6. "string.indexOf()" and "string.lastIndexOf()" vs regular expression

Methods string.indexOf() and string.lastIndexOf() are faster than regular expression and I recommend to use them for a simple search operations. Also it's helpful to know a differences between indexOf() and lastIndexOf() because here you can increase the performance, just think where is the most often search phrase.

7. Less code in the functions = better performance

Really nice post: https://top.fse.guru/nodejs-a-quick-optimization-advice-7353b820c92e. I want to note that this optimization is for V8 only. Our product SuperAdmin increases the max-inlined-source-size limit automatically for all Total.js applications.

8. "function.call" vs "function.apply"

function.call() is faster than function.apply() but both calls are much slower then calls without change of the context.