1 –
Don’t forget var keyword when assigning a variable’s value for the first time.
Assignment to an undeclared variable automatically results in a global variable being created. Avoid global variables.
2- Use strings accumulator-style: Using + operator a new string is created in memory and the concatenated value is assigned to it. Only after this the result is assigned to a variable.
Example:
a='A'+'B'; // slower
Faster :
a+='A';
a+='B';
3- Primitive operations can be faster than function calls: Consider using alternative primitive operation over function calls in performance critical loops and functions.
Slow:
var min = Math.min(a, b);
arr.push(val);
Faster:
var min = a < b ? a : b;
arr[arr.length] = val;
4-Maximize object resolution speed and minimize scope chain:
Inefficient way:
var url = location.href;
Efficient one:
var url = window.location.href;
5-use === instead of == : The == (or !=) operator performs an automatic type conversion if needed. The === (or !==) operator will not perform any conversion. It compares the value and the type, which could be considered faster than ==.
[10] === 10 // is false
[10] == 10 // is true
'10' == 10 // is true
'10' === 10 // is false
6-Use Semicolons for line termination: It is a best practice to use semi-colons for line termination.You won’t be warned if you forget it, because in most cases it will be inserted by the JavaScript parser.
7- Create a Self-calling Function:
(function(){
// some private code that will be executed automatically
})();
(function(a,b){
var result = a+b;
return result;
})(10,20)
this function
is self called function and the code will execute automatically.
8-Get a random number in a specific range:
var x = Math.floor(Math.random() * (max - min + 1)) + min;
9-Generate an array of numbers with numbers from 0 to maxvar numbersArray = [] , max = 100;
10-Append an array to another array:
var arr = [1 , "foo" , {name "Joe"} , -28];
var arr2 = ["Dog" , 5 , 100];
Array.prototype.push.apply(arr, arr2);
/* array1 will be equal to [1 , "foo" , {name "Joe"} , -28 , "Dog" , 5 , 100] */
11-Verify that a given argument is a number:
function isNumber(n){
return !isNaN(parseFloat(n)) && isFinite(n);
}
12-Verify that a given argument is an array:
Array.isArray(obj);
13-Get the max or the min in an array of numbers:
var numbers = [5, 4 , 12 , -21 , 28 , 400 , 122205, -8511];
var maximum = Math.max.apply(Math, numbers); // 122205
var minimum = Math.min.apply(Math, numbers); // -8511
14-Empty an array:
var myArray = [13 , 22 , 100 ];
myArray.length = 0; // myArray will be equal to [].
15-Don’t use delete to remove an item from array: Use splice instead of using delete to delete an item from an array. Using delete replaces the item with undefined instead of the removing it from the array.
16-Truncate an array using length:
var myArray = [12 , 222 , 1000 , 124 , 98 , 10 ];
myArray.length = 4; // myArray will be equal to [12 , 222 , 1000 , 124].
if you set the array length to a higher value, the length will be changed and new items will be added with undefined as a value. The array length is not a read only property.
myArray.length = 10; // the new array length is 10
myArray[myArray.length - 1] ; // undefined
17-Use the map() function method to loop through an array’s items:
var squares = [1,2,3,4].map(function (val) {
return val * val;
});
// squares will be equal to [1, 4, 9, 16]
18-Rounding number to N decimal place:
var num =2.443242342;
num = num.toFixed(4); // num will be equal to 2.4432
NOTE : the toFixed() function returns a string and not a number.
19-Floating point problems:
0.1 + 0.2 === 0.3 // is false
Why does this happen? 0.1 +0.2 is equal to 0.30000000000000004. What you need to know is that all JavaScript numbers are floating points represented internally in 64 bit.
20-Avoid negative indexes in arrays:
var numbersArray = [1,2,3,4,5];
var from = numbersArray.indexOf("foo") ; // from is equal to -1
numbersArray.splice(from,2); // will return [5]
if we using splice method with negative index then it will delete last element from the array.
21-Pass functions, not strings, to setTimeout() and setInterval():
If you pass a string into setTimeout() or setInterval(), the string will be evaluated the same way as with eval, which is slow.
Instead of using…
setInterval('doSomethingPeriodically()', 1000);
setTimeout('doSomethingAfterFiveSeconds()', 5000);
…use…
setInterval(doSomethingPeriodically, 1000);
setTimeout(doSomethingAfterFiveSeconds, 5000);
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