In-depth understanding of JavaScript study notes (1) Writing high-quality code_javascript skills

1. Variables

•Global variables
Two features of JavaScript make it surprisingly easy to create global variables unconsciously. First, you can use variables without even declaring them; second, JavaScript has an implicit global concept, which means that any variable you do not declare will become a global object property (not a true global variable, you can use delete Delete)

Copy code The code is as follows:

function sum(x,y) {
// result is not declared, it is an implicit global variable
result = x y;
return result;
}

function foo() {
// Use task chain for part var declaration, b is an implicit global variable
var a = b = 1;
}

Suggestion:

Copy Code The code is as follows:

function (x,y) {
var a ,b ;
a = b = 1;//a, b is a local variable
}

•var function
Global variables created through var (created in any program other than functions) cannot be deleted. Implicit global variables created without var (regardless of whether they are created in a function) can be deleted.

Copy code The code is as follows:

// Define three global variables
var global_var = 1;
global_novar = 2; // Negative teaching material
(function () {
global_fromfunc = 3; // Negative teaching material
}());

// Attempt to delete
delete global_var; // false
delete global_novar; // true
delete global_fromfunc; // true

// Test the delete
typeof global_var; // "number"
typeof global_novar; // "undefined"
typeof global_fromfunc; // "undefined"

•Single var declaration of variables
Use a single var statement at the top of the function It is a more useful form. The initial value of all uninitialized but declared variables is undefined

Copy code The code is as follows:

function func() {
var a = 1,
b = 2,
sum = a b,
myobject = {},
i,
j;
// function body...
}

•var spreading problem

Copy code The code is as follows:

//Counter example
myname = "global"; // Global variable
function func() {
alert(myname); //"undefined" var myname = "local"; alert(myname); // "local" } func(); is equivalent to:


myname = "global"; // global variable
function func( ) {
var myname; // Equivalent to -> var myname = undefined;
alert(myname); // "undefined"
myname = "local";
alert(myname); // "local"}
func();

Two for loops

• It is recommended to use

to copy Code The code is as follows:

function looper() {
var i = 0,
max,
myarray = [];
// ...
for (i = 0, max = myarray.length; i < max; i ) {
// Use myarray[i] to do something
}
}

Replace i with the following expression

Copy the code The code is as follows:

i = i 1
i = 1 The following two looping methods are faster


//The first changed form:

var i, myarray = [ ];
for (i = myarray.length; i–-;) {
// Use myarray[i] to do something
}

//The second one uses a while loop :

var myarray = [],
i = myarray.length;
while (i–-) {
// Do something with myarray[i]
}

•for-in loop
Apply to traversal of non-array objects. Arrays use normal for loops, and objects use for-in loops. Using the hasOwnProperty() method, you can filter out properties that come down the prototype chain when traversing object properties.

3 Avoid implicit type conversions

• Stick to === and! ==

4. Avoid using eval, and avoid passing strings to setInterval(), setTimeout() and Function() constructors, use functions instead.

Five parseInt() numerical conversion

It is recommended to assign a value to the base parameter,

Copy code The code is as follows:

var month = "06",
year = "09";
month = parseInt(month, 10);//Strings starting with 0 will be Treated as octal
year = parseInt(year, 10);

Six programming standards

Constructor naming: MyConstructor();

General Function naming: myFunction();

Variable naming: firstName;

Private properties or methods: _secondeName,

Constants: PI, MAX;