javascript what is a variable

In JavaScript, a variable is a "container" used to store information. The value is equivalent to what is contained in the container, and the variable name is the label attached to the container. The variable can be found through the label for reading and writing. The value it stores.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

A variable is equivalent to a container, and its value is equivalent to what is contained in the container. The variable name is the label attached to the container. The variable can be found through the label so that the value it stores can be read and written.

Declaring variables

In JavaScript, declare variables using the var statement.

Example 1

In a var statement, you can declare one or more variables, or assign values ​​to variables. Unassigned variables are initialized to undefined (undefined )value. When declaring multiple variables, they should be separated using the comma operator.

var a;  //声明一个变量
var a,b,c;  //声明多个变量
var b = 1; //声明并赋值
document.write(a);  //返回 undefined
document.write(b);  //返回 1

Example 2

In JavaScript, you can declare the same variable repeatedly, or you can initialize the value of the variable repeatedly.

var a = 1;
var a = 2;
var a = 3;
document.write(a);  //返回 3

Note:

In non-strict mode, JavaScript allows direct assignment of variables without declaring them. This is because the JavaScript interpreter can automatically declare variables implicitly. Implicitly declared variables are always used as global variables. In strict mode, variables must be declared before they can be used.

Assigning variables

Use the equal sign = operator to assign a value to a variable. The left side of the equal sign is the variable and the right side is the assigned value. value.

Example

Variable promotion. JavaScript will preprocess declared variables during precompilation, but the assignment of variables occurs during JavaScript execution, not during precompilation.

document.write(a); //显示undefined
a =1;
document.write(a); //显示 1
var a;

In the above example, the variable declaration is placed last and the assignment operation is placed first. Since JavaScript has pre-parsed the variable declaration statement during pre-compilation, the first line of code will not throw an exception when reading the variable value, but will return the uninitialized value undefined. The third line of code is read after the assignment operation, so it displays the number 1.

Tips:

The parsing method of the JavaScript engine is: first parse the code, obtain all declared variables, and then run it line by line. In this way, all declared variables will be hoisted to the head of the code, which is called variable hoisting (Hoisting).

【Related recommendations: javascript learning tutorial】

##Variable type

JavaScript is a weakly typed language, and the specifications for variable types are relatively loose. The specific performance is as follows:

• The type classification of variables is not rigorous and unclear, which leads to random use.

• When declaring a variable, it is not required to specify the type.

• The usage process is not strict and the variable type can be automatically converted as needed.

• There is no unified and standardized method for variable conversion and type checking, resulting in low development efficiency.

The resulting advantages and disadvantages are as follows:

• Advantages: Flexible use, simplifies code writing.

• Disadvantages: low execution efficiency, program performance will be affected when developing large applications.

For more programming-related knowledge, please visit:

Programming Video! !

The above is the detailed content of javascript what is a variable. For more information, please follow other related articles on the PHP Chinese website!