Optimization of repetitive operations in JavaScript

Often, novice programmers thoughtlessly call the same function several times, wasting resources.

Let's look at an example. Let's say we have a string containing some path:

javascriptlet path = 'img.png';

Suppose we are faced with the task of checking whether the path ends with the extension .png or .jpg. Some novice programmer wrote the following code:

javascriptlet path = 'img.png';

if (path.slice(-4) === '.png' || path.slice(-4) === '.jpg') {
  console.log('+++');
} else {
  console.log('---');
}

What is wrong with this solution? The problem is that the slice method is called twice, doing the same thing. This is, of course, not optimal.

To optimize, you need to slice the string once, write the result into a variable, and then use this variable further:

javascriptlet path = 'img.png';
let ext  = path.slice(-4);

if (ext === '.png' || ext === '.jpg') {
  console.log('+++');
} else {
  console.log('---');
}

javascriptlet num = 123;

if (String(num)[0] === '1' || String(num)[0] === '2') {
  console.log('+++');
} else {
  console.log('---');
}

javascriptlet date = new Date();

if (date.getDay() === 0 || date.getDay() === 6) {
  console.log('weekend');
} else {
  console.log('weekday');
}

javascriptlet date = new Date();
let res;

if (date.getFullYear() >= 2020 && date.getFullYear() <= 2030) {
  res = date.getFullYear() + ' fits';
} else {
  res = date.getFullYear() + ' doesn't fit';
}

console.log(res)