what is javascript var

var is a built-in keyword in javascript, used to declare variables and optionally initialize them to a value; the syntax format is "var varname [=value];", the parameter value can be any legal expression, the default value is "undefined".

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

Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is called "declaring" a variable.

You can declare JavaScript variables through the var keyword; the var statement declares a variable and optionally initializes it to a value:

var varname1 [= value1] [, varname2 [= value2] ... [, varnameN [= valueN]]];

• varname: variable name. Variable names can be defined as any legal identifier.

• value: The initialization value of the variable. The value can be any legal expression. The default value is undefined.

Description:

Variable declarations, wherever they occur, are processed before any code is executed. The scope of a variable declared with var is its current execution context, which can be a nested function, or globally for variables declared outside any function. If you redeclare a JavaScript variable, it will not lose its value.

When assigning a value to an undeclared variable, after the assignment is performed, the variable will be implicitly created as a global variable (it will become a property of the global object).

Example:

1. Declare and initialize two variables:

var a = 0, b = 0;

Assign the two variables to string values:

var a = "A";
var b = a;

// 等效于：
var a, b = a = "A";

Pay attention to the order:

var x = y, y = 'A';
console.log(x + y); // undefinedA

Here, x and y are created before the code is executed, and the assignment operation occurs after creation. When "x = y" is executed, y already exists, so no ReferenceError is thrown, and its value is 'undefined'. So x is assigned the value undefined. Then, y is assigned 'A'. Therefore, after executing the first line, x === undefined && y === 'A' resulted in this result.

[Related recommendations: javascript learning tutorial]

The difference between declared and undeclared variables is:

1. The scope of a declared variable is limited to the context of its declaration location, while an undeclared variable is always global.

function x() {
  y = 1;   // 在严格模式（strict mode）下会抛出 ReferenceError 异常
  var z = 2;
}

x();

console.log(y); // 打印 "1"
console.log(z); // 抛出 ReferenceError: z 未在 x 外部声明

2. Declared variables are created before any code is executed, while undeclared variables are only created when assignment operations are performed.

console.log(a);                // 抛出ReferenceError。
console.log('still going...'); // 打印"still going..."。
Copy to Clipboard
var a;
console.log(a);                // 打印"undefined"或""（不同浏览器实现不同）。
console.log('still going...'); // 打印"still going..."。

3. Declared variables are non-configurable properties of the context in which they are located, while non-declared variables are configurable (for example, non-declared variables can be deleted).

var a = 1;
b = 2;
delete this.a; // 在严格模式（strict mode）下抛出TypeError，其他情况下执行失败并无任何提示。
delete this.b;
console.log(a, b); // 抛出ReferenceError。
// 'b'属性已经被删除。

Because of these three differences, failure to declare a variable will most likely lead to unexpected results. Therefore, it is recommended to always declare variables, whether they are within a function or global scope. Also, in ECMAScript 5 strict mode, assigning to an undeclared variable throws an error.

For more programming related knowledge, please visit: Programming Video! !

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